#include <bits/stdc++.h>
int main() {
  int n, a[26] = {0}, j = 0, ok = 1;
  char s[501];
  scanf("%s", s);
  n = strlen(s);
  for (int i = 0; i < n; ++i) {
    if (s[i] - 'a' > j) {
      ok = 0;
      break;
    }
    if (a[s[i] - 'a'] == 0) {
      a[s[i] - 'a'] = 1;
      ++j;
    }
  }
  if (ok == 1)
    printf("YES");
  else
    printf("NO");
}
