#include <bits/stdc++.h>
using namespace std;
int n, mx = -1;
char arr[502];
int main() {
  scanf("%s", arr);
  n = strlen(arr);
  for (int i = 0; i < n; i++) {
    if (arr[i] - 'a' - 1 == mx) {
      mx = arr[i] - 'a';
    } else if (arr[i] - 'a' <= mx) {
      continue;
    } else {
      printf("NO\n");
      return 0;
    }
  }
  printf("YES\n");
  return 0;
}
