Let R_1 and R_2 be relations on the set {1,2, …., 50} such that R_1={(p, p^n): p. is a prime and n ≥ 0 is an integer } and R_2={(p, p^n): p. is a…

Let $\displaystyle R_1$ and $\displaystyle R_2$ be relations on the set $\displaystyle \{1,2, \ldots ., 50\}$ such that $\displaystyle R_1=\left\{\left(p, p^n\right): p\right.$ is a prime and $\displaystyle n \geq 0$ is an integer $\displaystyle \}$ and $\displaystyle R_2=\left\{\left(p, p^n\right): p\right.$ is a prime and $\displaystyle n=0$ or $\displaystyle 1 \}$. Then, the number of elements in $\displaystyle R_1-R_2$ is \_\_\_\_.