/in/foo.cc: In function 'int main()':
/in/foo.cc:18:43: error: 'abs' was not declared in this scope
     for (i=1; i<=n; ++i) ans += abs(y[i]-k);
                                           ^
/in/foo.cc:18:43: note: suggested alternative:
In file included from /in/foo.cc:2:0:
/usr/include/c++/6/cmath:103:5: note:   'std::abs'
     abs(_Tp __x)
     ^~~