#include <bits/stdc++.h>
using namespace std;
int main() {
  string a;
  cin >> a;
  int k = 0;
  char c = 'a';
  for (int i = 0; i < a.size(); i++) {
    if (a[i] > c) {
      k++;
      break;
    } else if (a[i] == c)
      c++;
  }
  if (k == 0)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
