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