Let R1 and R2 be relations on the set {1, 2, . . . , 50} such that R1 = {(p, pn) : p is a prime and n ≥ 0 is an integer} and R2 = {(p, pn) : p is a prime and n = 0 or 1}. Then, the number of elements in R1 - R2 is____.