foo.cc: In function 'int main()':
foo.cc:6:15: warning: unused variable 'sum2' [-Wunused-variable]
  int n,sum1=0,sum2=0,sum3=0;
               ^~~~
foo.cc:6:22: warning: unused variable 'sum3' [-Wunused-variable]
  int n,sum1=0,sum2=0,sum3=0;
                      ^~~~

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