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Question

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Johnny has some difficulty memorizing the small prime numbers. So, his computer science teacher has asked him to play with the following puzzle game frequently.

The puzzle is a 3x3 board consisting of numbers from 1 to 9. The objective of the puzzle is to swap the tiles until the following final state is reached:

1 2 3
4 5 6
7 8 9
At each step, Johnny may swap two adjacent tiles if their sum is a prime number. Two tiles are considered adjacent if they have a common edge.

Help Johnny to find the shortest number of steps needed to reach the goal state.
Input:
2

7 3 2
4 1 5
6 8 9

9 8 5
2 4 1
3 7 6

Output:
6
-1

Answer 1

When typed 2, from what moved to what and why?

Answer 2

i
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