#include <bits/stdc++.h>
using namespace std;
map<char, int> mp;
int main() {
  char c;
  string a;
  int i, res;
  while (cin >> a) {
    mp.clear();
    c = 'a';
    res = 0;
    for (i = 0; i < a.size(); i++) {
      if (mp[a[i]] == 1) continue;
      if (a[i] != c) {
        res = 1;
        break;
      } else {
        mp[a[i]] = 1;
        c++;
      }
    }
    if (res == 1)
      puts("NO");
    else
      puts("YES");
  }
  return 0;
}
