#include <bits/stdc++.h>   using namespace std;   int main() {     string s;     cin >> s;     char c =  a ;     for (int i = 0; i < s.size(); i++) {       if (s[i] ==  z  + 1)         continue;       else if (s[i] != c) {         cout <<  NO n ;         return 0;       } else {         for (int j = i; j < s.size(); j++) {           if (s[j] == c) s[j] =  z  + 1;         }         c++;       }     }     cout <<  YES n ;   }   