/in/foo.cc: In function 'std::__cxx11::string back(std::__cxx11::string, int)':
/in/foo.cc:16:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<str.length();i++){
              ~^~~~~~~~~~~~~
/in/foo.cc: In function 'std::__cxx11::string forw(std::__cxx11::string, int)':
/in/foo.cc:28:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<str.length();i++){
              ~^~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:46:6: warning: unused variable 'k' [-Wunused-variable]
  int k;
      ^