/in/foo.cc: In function 'int main()':
/in/foo.cc:12:22: warning: comparison of integer expressions of different signedness: 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   12 |         if (k.size() != len) {
      |             ~~~~~~~~~^~~~~~