输入描述:

The first line contains two integers $n (1\leq n \leq 8)$ and $k (1\leq k\leq 20)$, representing the length of the sequence and the difference value, respectively.
The second line contains $n$ integers $a_1, a_2, ..., a_n (1\leq a_i\leq 20)$, representing the elements in the sequence.

输出描述:

Output an integer in a single line, representing the number of permutations that achieve the goal.

3 2
1 2 4

2

In the example, only permutaions $[1, 4, 2]$ and $[2, 4, 1]$ achieve the goal.