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 _______________.