foo.cc: In function 'int main()':
foo.cc:8:2: error: 'cin' was not declared in this scope
  cin>>dan1;
  ^~~
foo.cc:8:2: note: suggested alternative:
In file included from foo.cc:2:0:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~

自豪的采用jd5进行评测(github.com/masnn/jd5)