#include <bits/stdc++.h>
using namespace std;
char a[12345];
int main() {
  int i, j, n, ans = 0, sum = 0, k = 0;
  scanf("%s", &a);
  if (a[0] == 'a') ans++;
  for (i = 1; i < strlen(a); i++) {
    if (a[i] - '0' <= a[k] - '0' || a[i] - '0' == a[k] - '0' + 1) {
      ans++;
      k = a[i] - '0' > a[k] - '0' ? i : k;
    }
  }
  if (ans == strlen(a))
    printf("YES\n");
  else
    printf("NO\n");
  return 0;
}
