foo.cc: In function 'int main()':
foo.cc:7:2: error: 'cin' was not declared in this scope
  cin>>a;
  ^
foo.cc:7:7: error: 'a' was not declared in this scope
  cin>>a;
       ^
foo.cc:8:7: error: 'b' was not declared in this scope
  cin>>b;
       ^

自豪的采用 HydroJudger 进行评测(github.com/hydro-dev/HydroJudger)