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