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