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