#include <bits/stdc++.h>
using namespace std;
void solve() {
  string s;
  cin >> s;
  int current = 0;
  for (int i = 0; i < s.length(); ++i) {
    if (s[i] == current + 'a') {
      ++current;
    } else if (s[i] > current + 'a') {
      cout << "NO\n";
      return;
    }
  }
  cout << "YES\n";
}
void reset() {}
int main() {
  int tests = 1;
  for (; tests; --tests) {
    solve();
    reset();
  }
}
