/in/foo.cc: In function 'char f(std::__cxx11::string)':
/in/foo.cc:6:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++){
              ~^~~~~~~~~~~
/in/foo.cc:10:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(sum==s.length())return 'I';
     ~~~^~~~~~~~~~~~