Lattice $B_2$

In two-dimensional Euclidean plane, let $\Lambda$ be the group generated by vectors $\lambda_1$ and $\lambda_2,\Lambda_r$ the group generated by vectors $\alpha_1$ and $\alpha_2$ (see the figure). $\Lambda=\{m\lambda_1+n\lambda_2\mid m,n\in\mathbb Z\}, \\\Lambda_r=\{m\alpha_1+n\alpha_2\mid m,n\in\mathbb Z\}.$

Clearly, $\Lambda_r$ is a subgroup of $\Lambda.$ Find the order of quotient group $\Lambda/\Lambda_r.$