#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  string ans = "YES";
  char lodka = 'a';
  cin >> s;
  for (auto ch : s) {
    if (ch < lodka) continue;
    if (ch > lodka) ans = "NO";
    if (ch == lodka) lodka++;
  }
  cout << ans << endl;
}
