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