/in/foo.cc: In function 'int main()':
/in/foo.cc:9:12: warning: unused variable 'j' [-Wunused-variable]
  int n, i, j, k, sum, a = 0, b = 0;
            ^
/in/foo.cc:9:15: warning: unused variable 'k' [-Wunused-variable]
  int n, i, j, k, sum, a = 0, b = 0;
               ^
/in/foo.cc:9:18: warning: unused variable 'sum' [-Wunused-variable]
  int n, i, j, k, sum, a = 0, b = 0;
                  ^~~