#include <bits/stdc++.h>
using namespace std;
int hcf(int a, int b) {
  if (!a)
    return (b);
  else
    return (hcf(b % a, a));
}
int lcm(int a, int b) { return ((a * b) / hcf(a, b)); }
long long isP(long long n) {
  if (n == 0 || n == 1) return 0;
  for (long long i = 2; i <= sqrt(n); i++) {
    if (n % i == 0) return 0;
  }
  return 1;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  string s;
  cin >> s;
  if (s[0] != 'a') {
    cout << "NO" << endl;
  } else {
    bool ans = false;
    for (long long i = 1; 1 < s.length() ? i < s.length() : i > s.length();
         1 < s.length() ? i += 1 : i -= 1) {
      bool ya = false;
      char a = s[i];
      if (s[i] == 'a') {
        a = 'b';
      }
      for (long long j = 0; j < i; j++) {
        if ((long long)s[j] == (long long)(a - 1)) {
          ya = true;
          break;
        }
      }
      if (ya == false) {
        cout << "NO" << endl;
        ans = true;
        break;
      }
    }
    if (ans == false) {
      cout << "YES" << endl;
    }
  }
  return 0;
}
