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