#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  char count = 'a';
  for (int i = 0; i < s.size(); ++i) {
    if (s[i] > count) {
      cout << "NO\n";
      return 0;
    } else if (s[i] == count) {
      count++;
    }
  }
  cout << "YES\n";
  return 0;
}
