#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  char x('a'), c;
  while (cin >> c) {
    if (c == x) ++x;
    if (c > x) {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
  return 0;
}
