foo.cc:3:1: error: expected identifier before 'int'
 int main(){
 ^~~
foo.cc:3:1: error: expected ';' before 'int'
foo.cc: In function 'int main()':
foo.cc:5:2: error: 'cin' was not declared in this scope
  cin >> n;
  ^~~
foo.cc:5:2: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
foo.cc:6:2: error: 'cout' was not declared in this scope
  cout << n << endl;
  ^~~~
foo.cc:6:2: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
foo.cc:6:15: error: 'endl' was not declared in this scope
  cout << n << endl;
               ^~~~
foo.cc:6:15: note: suggested alternative:
In file included from /usr/include/c++/7/iostream:39:0,
                 from foo.cc:1:
/usr/include/c++/7/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~

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