Compilare: main.cpp: In function 'std::string cer1(std::string)': main.cpp:17:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 17 | for(int i=0,ind=0;i<s.size();i+=2,ind++) | ~^~~~~~~~~