#include <bits/stdc++.h>
bool check(char* s) {
  for (int i = 0, c = 'a';; c++) {
    while (s[i] == '-') i++;
    if (s[i] == '\0')
      return true;
    else if (s[i] != c)
      return false;
    for (int j = i; s[j] != '\0'; j++)
      if (s[j] == c) s[j] = '-';
  }
}
int main() {
  char s[512];
  scanf("%s", s);
  printf(check(s) ? "YES\n" : "NO\n");
}
