/in/foo.cc: In function 'int main()':
/in/foo.cc:8:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<s.size();i++)
                 ~^~~~~~~~~
/in/foo.cc:11:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if((s.size()-i-1)%3==0&&i!=s.size()-1)cout<<',';
                                 ~^~~~~~~~~~~~