/in/foo.cc: In function 'std::__cxx11::string Concat(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:6:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<s1.size()-1;i++){
                 ~^~~~~~~~~~~~
/in/foo.cc:9:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=1;i<s2.size();i++){
                 ~^~~~~~~~~~