/in/foo.cc: In function 'void Output(std::vector<std::vector<int> >&)':
/in/foo.cc:27:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<M.size();i++)
              ~^~~~~~~~~
/in/foo.cc:29:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<M[0].size();j++)
               ~^~~~~~~~~~~~