Sets And Relations

Let \(A=\{2,3,6,7\}\) and \(B=\{4,5,6,8\}\). Let \(R\) be a relation defined on \(A \times B\) by \((a_1, b_1) R(a_2, b_2)\) if and only if \(a_1+a_2=b_1+b_2\). Then the number of elements in \(R\) is __________.