#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  string a;
  cin >> a;
  char next = 'a';
  for (char c : a) {
    if (c > next) {
      cout << "NO";
      return 0;
    }
    next = max(next, (char)(c + 1));
  }
  cout << "YES";
}
