/in/foo.cc: In function 'std::__cxx11::string n2()':
/in/foo.cc:25:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size() - len;) {
                  ~~^~~~~~~~~~~~~~~~
/in/foo.cc:33:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s1.size();) {
                  ~~^~~~~~~~~~~
/in/foo.cc:34:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (s1.size() - 4 > i && s1.substr(i, 4) == "1013") {
       ~~~~~~~~~~~~~~^~~
/in/foo.cc: In function 'std::__cxx11::string n3()':
/in/foo.cc:47:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size() - len; i++) {
                  ~~^~~~~~~~~~~~~~~~