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