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