/in/foo.cc: In function 'std::__cxx11::string add(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:23:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<max(a.length(),b.length());i++){
              ~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.cc:33:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=mm;i<mm+res.length();i++){
               ~^~~~~~~~~~~~~~~~