#include <bits/stdc++.h>
using namespace std;
const int N = 2e3 + 10;
int main() {
  string s;
  cin >> s;
  char a = 'a';
  for (int i = 0; i < s.size(); i++) {
    if (s[i] > a) {
      cout << "NO";
      return 0;
    }
    if (s[i] == a) a++;
  }
  cout << "YES";
}
