#include <bits/stdc++.h>
using namespace std;
using LD = long double;
int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout.setf(ios_base::fixed);
  cout.precision(28);
  int n;
  string s;
  cin >> s;
  bool ok = true;
  char ch = 'a';
  for (int i = 0; i < s.size(); ++i) {
    if (s[i] > ch) {
      ok = false;
    } else {
      if (s[i] == ch) {
        ch++;
      }
    }
  }
  if (ok) {
    cout << "YES";
    return 0;
  }
  cout << "NO";
  return 0;
}
