#include <bits/stdc++.h>
using namespace std;
const int MM = 200;
int main() {
  string str, tmp;
  cin >> str;
  char ch = 'a';
  while (true) {
    tmp = "";
    if (str[0] != ch) {
      cout << "NO" << endl;
      return 0;
    }
    for (int i = 0; i < str.size(); i++) {
      if (str[i] != ch) tmp += str[i];
    }
    str = tmp;
    if (str.empty()) break;
    ch++;
  }
  cout << "YES" << endl;
  return 0;
}
