/in/foo.cc: In function 'std::__cxx11::string get_k_str(std::__cxx11::string, std::__cxx11::string)':
/in/foo.cc:21:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=1;i<a.size();i++) {
                 ~^~~~~~~~~
/in/foo.cc:29:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++) {
                 ~^~~~~~~~~