/in/foo.cc: In function 'void px(std::vector<int>&)':
/in/foo.cc:7:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < v.size() - 1; i++) {
                  ~~^~~~~~~~~~~~~~
/in/foo.cc: In function 'int main(int, char**)':
/in/foo.cc:31:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < M.size(); i++) {
                  ~~^~~~~~~~~~