#include <bits/stdc++.h>
using namespace std;
string s;
int main() {
  cin >> s;
  int z = 0;
  for (int i = 0; i != s.size(); ++i) {
    if ((int)s[i] - 97 <= z) {
      if (s[i] - 97 == z) ++z;
    } else {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
  return 0;
}
