#include <bits/stdc++.h>
using namespace std;
int main(void) {
  string str;
  cin >> str;
  int i = -1;
  for (char c : str) {
    int j = c - 'a';
    if (i + 1 == j) {
      i = j;
    } else if (i < j) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
  return 0;
}
