Maximum join of relations

Question

I have a question as follows

Consider the relations $r1(P, Q, R)$ and $r2(R, S, T)$ with primary keys $P$ and $R$ respectively. The relation $r1$ contains $2000$ tuples and $r2$ contains $2500$ tuples. The maximum size of the join $r1⋈ r2$ is :

My attempt - Suppose all value of $R$ in $r1$ are same. Then it should be $4499$.

But it's given $2000$. Am I missing something?