/in/foo.cc: In function 'int main()':
/in/foo.cc:5:4: error: 'cin' was not declared in this scope
    cin >> n;
    ^~~
/in/foo.cc:5:11: error: 'n' was not declared in this scope
    cin >> n;
           ^
/in/foo.cc:7:2: error: 'arr' was not declared in this scope
  arr[0] = 1;
  ^~~
/in/foo.cc:18:2: error: 'cout' was not declared in this scope
  cout << sum << endl;
  ^~~~
/in/foo.cc:18:17: error: 'endl' was not declared in this scope
  cout << sum << endl;
                 ^~~~