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 Julia (1.4.0) (judge verdict: Wrong Answer)

const MOD = Int(10^9+7)

function solve()
    N, M = parse.(Int, split(readline()))
    a = zeros(Int, M)
    for i = 1:M
        a[i] = parse(Int, readline())
    end

    if N == 1
        println(1)
        return 0
    end

    dp = zeros(Int, N)
    dp[1] = 1
    dp[2] = 2

    for i = 1:M dp[a[i]] = -1 end

    for i = 2:N
        println(dp)
        if dp[i] < 0 continue end

        if i + 1 <= N
            if dp[i+1] >= 0
                dp[i+1] = (dp[i+1] + dp[i])%MOD
            end
        end

        if i + 2 <= N
            if dp[i+2] >= 0
                dp[i+2] = (dp[i+2] + dp[i])%MOD
            end
        end
    end

    println(dp[N])
end

solve()