foo.c: In function 'main':
foo.c:4:10: error: 'cin' undeclared (first use in this function); did you mean 'main'?
  int a,b;cin>>a>>b;
          ^~~
          main
foo.c:4:10: note: each undeclared identifier is reported only once for each function it appears in
foo.c:5:2: error: 'cout' undeclared (first use in this function)
  cout<<a+b;
  ^~~~

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