Compilare: main.cpp: In function 'int main()': main.cpp:15:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size()&&ok==1; ++i) ~^~~~~~~~~ main.cpp:16:15: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses] if(i==0 || i%2==0) ^