#include <bits/stdc++.h>
using namespace std;
int main(void) {
  string s;
  int x = 0, length = 0;
  cin >> s;
  int length2 = s.size();
  for (int i = 0; i < length2; i++) {
    x = (s[i] - 'a');
    if (x > length) {
      cout << "NO" << endl;
      return 0;
    }
    if (x == length) length++;
  }
  cout << "YES" << endl;
}
