#include <bits/stdc++.h>
using namespace std;
char str[(500 + 10)];
int maxs = 0;
int main() {
  scanf("%s", str);
  for (int i = 0; str[i] != '\0'; i++) {
    int x = str[i] - 'a';
    if (x == maxs) {
      maxs++;
    } else if (x > maxs) {
      printf("NO\n");
      return 0;
    }
  }
  printf("YES\n");
  return 0;
}
