#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int max = -1;
  bool is_bad = false;
  for (char x : s) {
    int p = x - 'a';
    if (max + 1 == p)
      max++;
    else if (p > max + 1) {
      is_bad = true;
      break;
    }
  }
  cout << (!is_bad ? "YES" : "NO") << endl;
  return 0;
}
