/in/foo.cc:23:11: error: '::main' must return 'int'
 void main()
           ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:26:9: error: 'cin' was not declared in this scope
         cin>>n>>m;
         ^~~
/in/foo.cc:26:9: note: suggested alternative: 'main'
         cin>>n>>m;
         ^~~
         main