#include <bits/stdc++.h>
using namespace std;
const int N = 1e3;
char a[N];
int main() {
  char c = 'a';
  gets(a);
  for (char *p = a; *p; ++p) {
    if (*p > c) {
      puts("NO");
      return 0;
    }
    if (*p == c) ++c;
  }
  puts("YES");
  return 0;
}
