foo.cc: In function 'int main()':
foo.cc:12:6: warning: unused variable 'ans' [-Wunused-variable]
  int ans=0;
      ^~~
foo.cc:13:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%s",ch);
  ~~~~~^~~~~~~~~

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