foo.cc: In function 'int main()':
foo.cc:11:9: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("sum.in","r",stdin);
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~
foo.cc:12:9: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("sum.out","w",stdout);
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
foo.cc:18:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a[tot]);
   ~~~~~^~~~~~~~~~~~~~

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