#include <bits/stdc++.h>
using namespace std;
const int MAX = 505;
int main() {
  char s[MAX];
  bool f[27];
  memset(f, false, sizeof(f));
  scanf("%s", &s);
  int i, len = strlen(s);
  for (i = 0; i < len; i++) {
    int temp = (s[i] - 'a');
    if (!temp)
      f[temp] = true;
    else if (!f[temp - 1]) {
      printf("NO\n");
      return 0;
    } else
      f[temp] = true;
  }
  printf("YES\n");
  return 0;
}
