/in/foo.cc: In function 'long long int cal(char*, int)':
/in/foo.cc:40:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < strlen(formula); ++i)
                  ~~^~~~~~~~~~~~~~~~~
/in/foo.cc: In function 'long long int cal_operator(long long int, long long int, char)':
/in/foo.cc:35:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^