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