#include <bits/stdc++.h>
using namespace std;
int a[30];
int main() {
  string s;
  cin >> s;
  int c = 1, mx = 0;
  for (int i = 0; i < s.size(); i++) {
    if (a[s[i] - 'a'] == 0) {
      a[s[i] - 'a'] = c;
      c++;
      mx = max(mx, s[i] - 'a');
    }
  }
  for (int i = 0; i <= mx; i++) {
    if (a[i] != i + 1) {
      cout << "NO\n";
      return 0;
    }
  }
  cout << "YES\n";
  return 0;
}
