| Time limit: | 1.00 s |
Memory limit: | 512 MB |
You know that an array has $n$ integers between $1$ and $m$, and the difference between two adjacent values is at most $1$.
Given a description of the array where some values may be unknown, your task is to count the number of arrays that match the description.
Input
The first input line has two integers $n$ and $m$: the array size and the upper bound for each value.
The next line has $n$ integers $x_1,x_2,\dots,x_n$: the contents of the array. Value $0$ denotes an unknown value.
Output
Print one integer: the number of arrays modulo $10^9+7$.
Constraints
- $1 \le n \le 10^5$
- $1 \le m \le 100$
- $0 \le x_i \le m$
Example
Input:
3 5
2 0 2
Output:
3
Explanation: The arrays $[2,1,2]$, $[2,2,2]$ and $[2,3,2]$ match the description.
