At time t=0,
A has queue contain packets 1, 2, 3, 4. As A - R link is instantaneous, this window is immediately available at R. 1 starts travelling from R. Now window contains packets 2, 3, 4.
At time t=1,
1 reaches B. Ack starts from B to R. 2 starts travelling from A. Now window contains 3, 4.
At time t = 2,
2 reaches B. Ack for 1 reaches R. Which is immediately available at A. Then window slides left. Now packets 3, 4, 5 are in queue. 3 starts travelling from A. Then packets 4 & 5 are in queue.
At time t = 3,
3 reaches B. Ack for 2 reaches R. Which is immediately available at A. Then window slides left. Now packets 4, 5, 6 are in queue. 4 starts travelling from A. Then packets 5 & 6 are in queue.
At time t = 4,
4 reaches B. Ack for 3 reaches R. Which is immediately available at A. Then window slides left. Now packets 5, 6, 7 are in queue. 5 starts travelling from A. Then packets 6 & 7 are in queue.
At any time a+1, a+2 are waiting in queue.