Double factorial

The double factorial $n!!$ is the product of numbers from $n$ downwards in steps of two: $7!! = 7 \cdot 5 \cdot 3 \cdot 1 = 105$, $8!! = 8 \cdot 6 \cdot 4 \cdot 2 = 384$. $0!! = 1!! = 1$. Compute $n!!$ with a recursive function.

A single integer $n$ ($0 \le n \le 33$).

A single integer — $n!!$.

Recursive step: $n!! = n \cdot (n - 2)!!$. There are two base cases: $0!! = 1$ (even $n$ ends here) and $1!! = 1$ (odd $n$). They can be merged into "if $n \le 1$, return $1$". $33!!$ still fits in a 64-bit type.