foo.cc: In function ‘int main()’:
foo.cc:19:25: warning: format ‘%s’ expects argument of type ‘char*’, but argument 2 has type ‘std::string’ {aka ‘std::__cxx11::basic_string<char>’} [-Wformat=]
   19 |                 scanf("%s", s1);
      |                        ~^
      |                         |
      |                         char*
foo.cc:20:25: warning: format ‘%s’ expects argument of type ‘char*’, but argument 2 has type ‘std::string’ {aka ‘std::__cxx11::basic_string<char>’} [-Wformat=]
   20 |                 scanf("%s", s2);
      |                        ~^
      |                         |
      |                         char*