#include <bits/stdc++.h>
using namespace std;
int vis[505];
int main() {
  string s;
  cin >> s;
  int cnt = 0, temp = 0;
  while (cnt < s.size()) {
    int i = 0;
    for (i = 0; i < s.size() && vis[i]; i++)
      ;
    char mu = 'a' + temp;
    if (s[i] != mu) {
      cout << "NO" << endl;
      return 0;
    }
    temp++;
    for (; i < s.size(); i++) {
      if (s[i] == mu) {
        vis[i] = 1;
        cnt++;
      }
    }
  }
  cout << "YES" << endl;
}
