#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  char prev = '`';
  for (int i = 0; i < s.size(); i++) {
    if (s[i] - prev > 1) {
      cout << "NO";
      return 0;
    }
    prev = max(prev, s[i]);
  }
  cout << "YES";
  return 0;
}
