#include <bits/stdc++.h>
using namespace std;
bool valid(char str[]) {
  int now = 0;
  for (int i = 0; str[i]; i++) {
    if (str[i] > now + 'a') return false;
    if (str[i] == now + 'a') now++;
  }
  return true;
}
int main() {
  char in[550];
  scanf("%s", in);
  int mx = 0;
  for (int i = 0; in[i]; i++) mx = max(mx, in[i] - 'a');
  int cnt = 0;
  if (valid(in))
    printf("YES");
  else
    printf("NO");
}
