Problem:
Score : 300 points

Problem Statement

There is a staircase with N steps. Takahashi is now standing at the foot of the stairs, that is, on the 0-th step.
He can climb up one or two steps at a time.

However, the treads of the a_1-th, a_2-th, a_3-th, \ldots, a_M-th steps are broken, so it is dangerous to set foot on those steps.

How many are there to climb up to the top step, that is, the N-th step, without setting foot on the broken steps?
Find the count modulo 1\ 000\ 000\ 007.

Constraints

1 \leq N \leq 10^5

0 \leq M \leq N-1

1 \leq a_1 < a_2 < ... < a_M \leq N-1

Input

Input is given from Standard Input in the following format:

N M
a_1
a_2
.
.
.
a_M

Output

Print the number of ways to climb up the stairs under the condition, modulo 1\ 000\ 000\ 007.

Sample Input 1

6 1
3

Sample Output 1

4

There are four ways to climb up the stairs, as follows:

0 \to 1 \to 2 \to 4 \to 5 \to 6

0 \to 1 \to 2 \to 4 \to 6

0 \to 2 \to 4 \to 5 \to 6

0 \to 2 \to 4 \to 6

Sample Input 2

10 2
4
5

Sample Output 2

0

There may be no way to climb up the stairs without setting foot on the broken steps.

Sample Input 3

100 5
1
23
45
67
89

Sample Output 3

608200469

Be sure to print the count modulo 1\ 000\ 000\ 007.

Solution in Kotlin (1.0.0) (judge verdict: Runtime Error)

import java.util.*
fun main(args:Array<String>) {
    val (N,M) = readLine()!!.split(" ").map(String::toInt)
    var step_list = Array(N) {0L}
    (1..M).forEach {
        val i = readLine()!!.toInt()
        step_list[i - 1] = -1L
    }
    step_list[0] = if(step_list[0] < 0L) {step_list[0]} else {1L}
    step_list[1] = if(step_list[1] < 0L) {step_list[1]} else {if(step_list[0] <0L) {1L} else {2L}}

    for (i in 2.until(N)) {
        if(step_list[i] >= 0L) {
            step_list[i] = if(step_list[i-2] < 0L && step_list[i - 1] < 0L) {
                -1L 
            } else {
                arrayOf(step_list[i - 2], step_list[i - 1]).filter {it >= 0L}.fold(0L) {sum,ele -> (sum % 1000000007L + ele % 1000000007L) % 1000000007L}
            } 
        }

    }
    arrayOf(0L,step_list[N-1]).max()!!.run(::println)
}