#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int m, k = 1, i;
  for (i = 0; i < s.size(); i++) {
    m = int(s[i]) - 96;
    if (m == k)
      k++;
    else if (m > k) {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
}
