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