#include <bits/stdc++.h>
using namespace std;
int main() {
  string str;
  vector<bool> f(26, false);
  cin >> str;
  if (str[0] == 'a') {
    f[0] = true;
    for (int i = 1; i < str.size(); ++i) {
      if (f[str[i] - 'a']) continue;
      if (!f[str[i] - 'a' - 1]) {
        cout << "NO";
        return 0;
      } else
        f[str[i] - 'a'] = true;
    }
    cout << "YES";
  } else
    cout << "NO";
}
