R$11.$ Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host are $R_1$ and $R_2,$ respectively. Assuming that the switch uses store$-$and$-$forward packet switching, what is the total end$-$to$-$end delay to send a packet of length L$?($Ignore queuing, propagation delay, and processing delay.$)$