#include <bits/stdc++.h>
using namespace std;
char s[505];
int main() {
  cin >> s;
  int sl = strlen(s);
  char c = 'a';
  if (s[0] != c) {
    cout << "NO" << endl;
    return 0;
  }
  for (int i = 1; i < sl; i++) {
    if (s[i] == c + 1)
      c++;
    else if (s[i] > c + 1) {
      cout << "NO" << endl;
      return 0;
    }
  }
  cout << "YES" << endl;
}
