/in/foo.cc: In function 'int main()':
/in/foo.cc:18:24: error: 'res' was not declared in this scope
      if (i * (i - 1) < res && res <= (i + 1) * i) {
                        ^~~
/in/foo.cc:17:10: warning: unused variable 'res2' [-Wunused-variable]
      int res2 = 2 * n;
          ^~~~
/in/foo.cc:32:21: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long long int' [-Wformat=]
     printf("%d", ans);
                     ^