#include <bits/stdc++.h>
using namespace std;
int n, m, i, j, k, x, y, z;
char s[510], now, cur;
int main() {
  cin >> s;
  n = strlen(s);
  for (i = 0; i < n; i++) {
    cur = s[i] - 'a';
    if (now == cur)
      now++;
    else if (now < cur) {
      cout << "NO";
      return 0;
    }
  }
  cout << "YES";
  return 0;
}
