/in/foo.cc: In function 'std::__cxx11::string inverse(std::__cxx11::string)':
/in/foo.cc:11:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0 ; i < a.length()/2 ; i ++){
                  ~~^~~~~~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string add(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:27:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0 ; i < a.length() ; i++){
                  ~~^~~~~~~~~~~~
/in/foo.cc:32:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = a.length() ; i <b.length() ; i++){
                           ~~^~~~~~~~~~~