#include <bits/stdc++.h>
const int INF = 0x3f3f3f3f;
using namespace std;
int main() {
  string s;
  cin >> s;
  vector<int> v(26, INF);
  for (int i = 0; i < s.size(); i++) {
    int k = s[i] - 'a';
    v[k] = min(i, v[k]);
  }
  for (int i = 1; i < 26; i++) {
    if (v[i] < v[i - 1]) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
  return 0;
}
