#include <bits/stdc++.h>
using namespace std;
int n;
char s[505];
int pos[26];
int main() {
  scanf("%s\n", s + 1);
  n = strlen(s + 1);
  for (int i = 0; i < 26; ++i) pos[i] = 0x3f3f3f3f;
  for (int i = 1; i <= n; ++i)
    if (pos[s[i] - 'a'] == 0x3f3f3f3f) pos[s[i] - 'a'] = i;
  for (int i = 0; i < 25; ++i)
    if (pos[i] > pos[i + 1]) return 0 * printf("NO\n");
  printf("YES\n");
  return 0;
}
