#include <bits/stdc++.h>
using namespace std;
int main() {
  char ref = 'a';
  char x;
  bool end = false;
  while (!end and cin >> x) {
    if (x > ref)
      end = true;
    else if (x == ref)
      ++ref;
  }
  if (end)
    cout << "NO" << endl;
  else
    cout << "YES" << endl;
}
