foo.cc:5:1: error: expected initializer before 'using'
 using namespace std;
 ^~~~~
foo.cc: In function 'int main()':
foo.cc:10:2: error: 'cin' was not declared in this scope
  cin >> a >> endl;
  ^~~
foo.cc:10:2: 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:10:14: error: 'endl' was not declared in this scope
  cin >> a >> endl;
              ^~~~
foo.cc:10:14: 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)
     ^~~~
foo.cc:11:8: error: 'jisuandigui' was not declared in this scope
  int b=jisuandigui(a)
        ^~~~~~~~~~~
foo.cc:11:6: warning: unused variable 'b' [-Wunused-variable]
  int b=jisuandigui(a)
      ^

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