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