/in/foo.cc: In function 'int main()':
/in/foo.cc:7:2: error: 'cin' was not declared in this scope
  cin>>a;
  ^~~
/in/foo.cc:7:2: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
/in/foo.cc:8:12: error: request for member 'length' in 'a', which is of non-class type 'char [800]'
  int len=a.length
            ^~~~~~
/in/foo.cc:10:8: error: 'left' was not declared in this scope
  while(left<=right)
        ^~~~
/in/foo.cc:10:8: note: suggested alternative:
In file included from /usr/include/c++/6/ios:42:0,
                 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/ios_base.h:999:3: note:   'std::left'
   left(ios_base& __base)
   ^~~~
/in/foo.cc:10:14: error: 'right' was not declared in this scope
  while(left<=right)
              ^~~~~
/in/foo.cc:10:14: note: suggested alternative:
In file included from /usr/include/c++/6/ios:42:0,
                 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/ios_base.h:1007:3: note:   'std::right'
   right(ios_base& __base)
   ^~~~~
/in/foo.cc:13:51: error: 'n' was not declared in this scope
   if(a[mid]*a[mid]*a[mid]+a[mid]*a[mid]+3*a[mid]<=n&&a[mid+1]*a[mid+1]*a[mid+1]+a[mid+1]*a[mid+1]+3*a[mid+1]>n)
                                                   ^
/in/foo.cc:23:2: error: 'cout' was not declared in this scope
  cout<<p;
  ^~~~
/in/foo.cc:23:2: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
/in/foo.cc:8:6: warning: unused variable 'len' [-Wunused-variable]
  int len=a.length
      ^~~
/in/foo.cc:25:1: error: expected '}' at end of input
 }
 ^