#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[505];
  scanf("%s", s);
  int n = strlen(s), m = -1, f = 1;
  for (int i = 0; i < n; i++) {
    int x = s[i] - 'a';
    if (x > m + 1) {
      f = 0;
      break;
    } else if (x == m + 1)
      m++;
  }
  if (f)
    printf("YES\n");
  else
    printf("NO\n");
  return 0;
}
