#include <bits/stdc++.h>
using namespace std;
int main() {
  bool k = true;
  int ma = -1;
  string s;
  cin >> s;
  for (int i = 0; i < s.length(); ++i) {
    if (s[i] - 'a' > ma + 1) {
      k = false;
      break;
    }
    if (s[i] - 'a' > ma) ma++;
  }
  if (k == false)
    cout << "NO" << endl;
  else
    cout << "YES" << endl;
  return 0;
}
