#include <bits/stdc++.h>
using namespace std;
int main() {
  char a[5000];
  scanf("%s", a);
  getchar();
  if (a[0] != 'a') {
    printf("NO");
    return 0;
  }
  char max = 'a';
  int logo = 0;
  for (int i = 1; a[i] != 0; i++) {
    if (a[i] == max + 1) {
      max = a[i];
    } else if (a[i] <= max) {
      continue;
    } else {
      logo = 1;
      break;
    }
  }
  if (logo == 1)
    printf("NO");
  else
    printf("YES");
}
