foo.cc: In function 'int main()':
foo.cc:37:5: warning: unused variable 'df' [-Wunused-variable]
 int df=0;
     ^~
foo.cc:36:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d",&n,&m);
  ~~~~~^~~~~~~~~~~~~~

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