/in/foo.cc: In function 'int main()':
/in/foo.cc:50:9: error: no match for 'operator[]' (operand types are 'std::vector<bool>' and '<unresolved overloaded function type>')
    flags[index] = true;
         ^
In file included from /usr/include/c++/6/vector:65:0,
                 from /usr/include/c++/6/queue:61,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:86,
                 from /in/foo.cc:2:
/usr/include/c++/6/bits/stl_bvector.h:854:5: note: candidate: std::vector<bool, _Alloc>::reference std::vector<bool, _Alloc>::operator[](std::vector<bool, _Alloc>::size_type) [with _Alloc = std::allocator<bool>; std::vector<bool, _Alloc>::reference = std::_Bit_reference; std::vector<bool, _Alloc>::size_type = long unsigned int]
     operator[](size_type __n)
     ^~~~~~~~
/usr/include/c++/6/bits/stl_bvector.h:854:5: note:   no known conversion for argument 1 from '<unresolved overloaded function type>' to 'std::vector<bool>::size_type {aka long unsigned int}'
/usr/include/c++/6/bits/stl_bvector.h:861:5: note: candidate: std::vector<bool, _Alloc>::const_reference std::vector<bool, _Alloc>::operator[](std::vector<bool, _Alloc>::size_type) const [with _Alloc = std::allocator<bool>; std::vector<bool, _Alloc>::const_reference = bool; std::vector<bool, _Alloc>::size_type = long unsigned int]
     operator[](size_type __n) const
     ^~~~~~~~
/usr/include/c++/6/bits/stl_bvector.h:861:5: note:   no known conversion for argument 1 from '<unresolved overloaded function type>' to 'std::vector<bool>::size_type {aka long unsigned int}'
/in/foo.cc:37:6: warning: unused variable 'sum' [-Wunused-variable]
  int sum = 0;
      ^~~