A frog starts by jumping onto stone . It then jumps between the stones, only ever jumping to adjacent ones. For fixed , it makes exactly visits to each stone for ; however, there are no restrictions on the number of times stone is visited. The frog can finish on any stone.
If and it would visit stone two times, stone three times and stone four times.
One way of achieving this is: .
Let be the number of ways the frog can make such a journey.
For example, , and .
Find . Give your answer modulo .
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