#include <bits/stdc++.h>
using namespace std;
int gcd(int a, int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
int lcm(int a, int b) { return a * (b / gcd(a, b)); }
long long powi(long long base, long long exp, long long m) {
  long long res = 1;
  while (exp) {
    if (exp % 2) {
      res *= base;
      res %= m;
    }
    base *= base;
    base %= m;
    exp >>= 1;
  }
  return res % m;
}
int main(int argc, const char* argv[]) {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  int freq[27] = {0};
  for (int i = 0; i < s.size(); i++) {
    for (int j = 0; j < s[i] - 'a'; j++) {
      if (freq[j] == 0) {
        cout << "NO";
        return 0;
      }
    }
    freq[s[i] - 'a']++;
  }
  cout << "YES";
  return 0;
}
