OBM 2001 Problem 2

Given $a_0 > 1$ , the sequence $a_0, a_1, a_2, ...$ is such that for all $k > 0$ , $a_k$ is the smallest integer greater than $a_{k-1}$ which is relatively prime to all the earlier terms in the sequence. Find all $a_0$ for which all terms of the sequence are primes or prime powers.