40分，求助！AC必关！

#include <bits/stdc++.h>
using namespace std;
int main (){
    string s;
    cin >> s;
    char a = s [0];
    if (! ((a >= 'A' && a <= 'Z') || (a >= 'a' && a <= 'z') || a == '_'))
        return printf ("no"), 0;
    for (int i = 1; i <= s.length (); i ++)
    {
        char c = s [i];
        bool Letter = (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z'); 
        bool Digit = (c >= '0' && c <= '9'); 
        bool Under = (c == '_'); 
        if (! (Letter || Digit || Under))
            return printf ("no"), 0;
    }
    return printf ("yes") ,0;
}