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