#include <bits/stdc++.h>
using namespace std;
string s;
int alpha[26];
int flag = 0;
int main() {
  cin >> s;
  int len = s.length();
  for (int i = 0; i < len; ++i) {
    alpha[s[i] - 'a'] = 1;
    for (int j = 0; j < (s[i] - 'a'); j++) {
      if (alpha[j] == 0) {
        cout << "NO";
        flag = 1;
        break;
      }
    }
    if (flag == 1) break;
  }
  if (flag == 0) cout << "YES";
  return 0;
}
