#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  t = 1;
  while (t--) {
    string s;
    cin >> s;
    set<int> a;
    a.insert(-1);
    int c = 0;
    for (int i = 0; i < s.length(); i++) {
      if (((s[i] - 'a') - (*a.rbegin())) == 1)
        a.insert(s[i] - 'a');
      else if (((s[i] - 'a') - (*a.rbegin())) <= 0)
        continue;
      else {
        c = 1;
        break;
      }
    }
    if (c == 0)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
  return 0;
}
