#include <bits/stdc++.h>
using namespace std;
bool comp(pair<long long int, long long int> v1,
          pair<long long int, long long int> v2) {
  if (v1.first == v2.first) {
    return v1.second > v2.second;
  }
  return v1.first > v2.first;
}
int main() {
  long long int n, i, j, k, flag;
  string s;
  cin >> s;
  map<long long int, long long int> mp;
  for (i = 1; i <= 26; i++) {
    mp[i] = 0;
  }
  for (i = 0; i < s.length(); i++) {
    k = (s[i] - 'a') + 1;
    mp[k] = 1;
    for (j = 1; j <= k; j++) {
      if (mp[j] == 0) {
        cout << "NO";
        return 0;
      }
    }
  }
  cout << "YES";
}
