/in/foo.cc: In function 'int main()':
/in/foo.cc:19:18: error: no match for 'operator<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'const char [3]')
  if(a[i]!=n) cout<"-1";
              ~~~~^~~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:962:5: note: candidate: template<class _BiIter> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, const std::__cxx11::sub_match<_BiIter>&)
     operator<(const sub_match<_BiIter>& __lhs, const sub_match<_BiIter>& __rhs)
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:962:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:1041:5: note: candidate: template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&, const std::__cxx11::sub_match<_BiIter>&)
     operator<(const __sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1041:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:1121:5: note: candidate: template<class _Bi_iter, class _Ch_traits, class _Ch_alloc> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, std::__cxx11::__sub_match_string<_Bi_iter, _Ch_traits, _Ch_alloc>&)
     operator<(const sub_match<_Bi_iter>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1121:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:1198:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type*, const std::__cxx11::sub_match<_BiIter>&)
     operator<(typename iterator_traits<_Bi_iter>::value_type const* __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1198:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'const char [3]'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:1272:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type*)
     operator<(const sub_match<_Bi_iter>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1272:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:1349:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const typename std::iterator_traits<_Iter>::value_type&, const std::__cxx11::sub_match<_BiIter>&)
     operator<(typename iterator_traits<_Bi_iter>::value_type const& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1349:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   mismatched types 'const std::__cxx11::sub_match<_BiIter>' and 'const char [3]'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/regex:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:110,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/regex.h:1429:5: note: candidate: template<class _Bi_iter> bool std::__cxx11::operator<(const std::__cxx11::sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)
     operator<(const sub_match<_Bi_iter>& __lhs,
     ^~~~~~~~
/usr/include/c++/6/bits/regex.h:1429:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'const std::__cxx11::sub_match<_BiIter>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/istream:38,
                 from /usr/include/c++/6/sstream:38,
                 from /usr/include/c++/6/complex:45,
                 from /usr/include/c++/6/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:52,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_pair.h:441:5: note: candidate: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&)
     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
     ^~~~~~~~
/usr/include/c++/6/bits/stl_pair.h:441:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'const std::pair<_T1, _T2>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/istream:38,
                 from /usr/include/c++/6/sstream:38,
                 from /usr/include/c++/6/complex:45,
                 from /usr/include/c++/6/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:52,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:298:5: note: candidate: template<class _Iterator> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator<(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:298:5: note:   template argument deduction/substitution failed:
/in/foo.cc:19:19: note:   'std::ostream {aka std::basic_ostream<char>}' is not derived from 'const std::reverse_iterator<_Iterator>'
  if(a[i]!=n) cout<"-1";
                   ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/istream:38,
                 from /usr/include/c++/6/sstream:38,
                 from /usr/include/c++/6/complex:45,
                 from /usr/include/c++/6/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:52,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator.h:355:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_it