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