foo.cc: In function 'int main()':
foo.cc:7:12: warning: unused variable 'i' [-Wunused-variable]
  int a,b,c,i,n1,n2,sum;cin>>n1>>n2;
            ^
foo.cc:7:20: warning: unused variable 'sum' [-Wunused-variable]
  int a,b,c,i,n1,n2,sum;cin>>n1>>n2;
                    ^~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。