Let d be an integer,not a perfect square.Let K=Q(sqrt(d)),and O_K umed to be UFD, be ring of integers of K.?

Let d be an integer, not a perfect square. Let K = Q(\sqrt(d)), and O_K umed to be UFD, be the ring of integers of K. Show that an odd rational prime p is either a prime in O_K or the product of at most two primes of O_K, which then are necessarily conjugates.