First Shared Stop On Two Delivery Trails

Two autonomous delivery drones, A and B, each fly a fixed, ordered route of checkpoints from their own warehouse to a shared drop site. Every checkpoint on either route is identified by a distinct positive integer ID. It sometimes happens that, from some point onward, drone A's remaining checkpoints and drone B's remaining checkpoints are exactly the same sequence of IDs in the same order, all the way to the drop site -- that is, the two routes merge onto a common trailing path. Given both complete routes, find the ID of the very first checkpoint at which the two routes merge, or determine that they never merge.

Input format

The first line contains two integers n and m (1 <= n, m <= 20000) -- the number of checkpoints on drone A's route and on drone B's route. The second line contains n space-separated integers (1 <= id <= 10^9): drone A's checkpoint IDs, in the order it visits them. The third line contains m space-separated integers (1 <= id <= 10^9): drone B's checkpoint IDs, in the order it visits them. It is guaranteed that either (a) the two routes share no checkpoint at all, or (b) there exists an integer k >= 1 such that the last k IDs of A's route are, in order, identical to the last k IDs of B's route -- and that in either case, no checkpoint ID outside of that shared trailing path ever repeats anywhere in the combined input.

Output format

Print a single integer: the ID of the first checkpoint of the shared trailing path (i.e. drone A's route's ID at position n-k), or -1 if the two routes never merge.

Constraints

• 1 <= n, m <= 20000
• 1 <= id <= 10^9
• The uniqueness / shared-tail guarantee above always holds for the given input.

5 4
2 7 11 4 1
9 11 4 1

11

3 3
1 2 3
4 5 6

-1