/in/foo.cc: In function 'int main()':
/in/foo.cc:30:5: error: 'cin' was not declared in this scope
     cin >> a >> b ; a -- ;
     ^~~
/in/foo.cc:36:33: error: 'sort' was not declared in this scope
     sort(w + 1 , w + n + 1 , cmp);
                                 ^
/in/foo.cc:39:5: error: 'cout' was not declared in this scope
     cout << b - a - ans;
     ^~~~