#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s;
  cin >> s;
  char mx = s[0];
  if (mx != 'a') {
    cout << "NO\n";
    return 0;
  }
  bool ok = true;
  for (int i = 1; i < s.size(); i++) {
    if (s[i] > mx + 1) {
      ok = false;
      break;
    }
    mx = max(mx, s[i]);
  }
  cout << (ok ? "YES\n" : "NO\n");
  return 0;
}
