foo.cc: In function 'int main()':
foo.cc:11:3: error: 'sum' was not declared in this scope
   sum+=(i-m)*a[i];
   ^~~
foo.cc:15:17: error: 'sum' was not declared in this scope
   long long now=sum+(i-m)*s2;
                 ^~~

[Hydro](https://hydro.ac)提供评测服务