运行 ID: 4988

Main.cc: In function ‘int main()’:
Main.cc:5:5: error: ‘cin’ was not declared in this scope
     cin>>r;
     ^~~
Main.cc:5:5: note: suggested alternative:
In file included from Main.cc:1:0:
/usr/include/c++/7/iostream:60:18: note:   ‘std::cin’
   extern istream cin;  /// Linked to standard input
                  ^~~