Determine all pairs (k, n) such that there exists a k-regular graph on n vertices

Question

I know that "a graph G is called k-regular if all its vertices have degree exactly k"

But I don't know to proceed towards the solution.

Question- Determine all pairs (k, n) such that there exists a k-regular graph on n vertices

Answer 1

Hint: Given an $n$, the value of $nk$ is even.