foo.cc: In function 'int main()':
foo.cc:6:21: warning: unused variable 'sum2' [-Wunused-variable]
  long long n,sum1=0,sum2=0,sum3=0;
                     ^~~~
foo.cc:6:28: warning: unused variable 'sum3' [-Wunused-variable]
  long long n,sum1=0,sum2=0,sum3=0;
                            ^~~~

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