#include <bits/stdc++.h>
using namespace std;
const int N = 1000006, LG = 21, mod = 1e9 + 7, M = N;
const double eps = 1e-3, pi = acos(-1.0);
const long long INF = 1123456789123456789LL;
int main() {
  string str;
  cin >> str;
  char mi = 'a';
  bool ans = true;
  for (char c : str)
    if (c > mi + 1) {
      ans = false;
      break;
    } else
      mi = max(mi, c);
  if (ans && str[0] == 'a')
    printf("YES\n");
  else
    printf("NO\n");
}
