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 C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        const long INF = 1L << 60;
        var a = new HashSet<int>();
        var dp = new long[1000010];

        var input = Console.ReadLine().Split(" ").Select(int.Parse).ToArray();
        int N = input[0], M = input[1];

        for (var i = 0; i < M; i++) a.Add(int.Parse(Console.ReadLine()));
        for (var i = 0; i < 1000010; i++) dp[i] = 0;

        dp[0] = 1;

        for (var i = 1; i <= N; i++)
        {
            if (a.Contains(i)) continue;

            if (i > 1 && !a.Contains(i - 1) && !a.Contains(i - 2))
            {
                dp[i] = dp[i - 2] + dp[i - 1];
            } else if (i > 1 && !a.Contains(i - 2))
            {
                dp[i] = dp[i - 2];
            } else if (!a.Contains(i - 1))
            {
                dp[i] = dp[i - 1];
            }
        }
        
        Console.WriteLine(dp[N] % 1000000007);
    }
}