#include <bits/stdc++.h>
using namespace std;
int main() {
  string data;
  cin >> data;
  int n = data.length();
  map<char, int> pos;
  for (int i = 0; i < n; i++) pos.insert({data[i], i});
  char c = 'a';
  int s = -1;
  for (auto i : pos) {
    if (i.first != c || i.second <= s) {
      printf("NO");
      return 0;
    }
    c++;
    s = i.second;
  }
  printf("YES");
  return 0;
}
