foo.cc: In function ‘int calc(int)’:
foo.cc:6:13: warning: unused variable ‘res’ [-Wunused-variable]
    6 |         int res = 0;
      |             ^~~
foo.cc: In function ‘int main()’:
foo.cc:25:19: warning: unused variable ‘ans’ [-Wunused-variable]
   25 |         long long ans=0;
      |                   ^~~
foo.cc:26:13: warning: unused variable ‘pre’ [-Wunused-variable]
   26 |         int pre=0;
      |             ^~~