/in/foo.cc: In function 'bool can(int)':
/in/foo.cc:8:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    8 |     if(a==0||a==s1.size()-1||s1[a]==s1[a-1]||s1[a]==s1[a+1])return 1;
      |              ~^~~~~~~~~~~~~