/in/foo.cc: In function 'int main()':
/in/foo.cc:28:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
          for(int i=0;i<s.size();i++) 
                      ~^~~~~~~~~
/in/foo.cc:12:10: warning: unused variable 'N' [-Wunused-variable]
    int M,N,t;
          ^