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# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
public class ABC
{
    public static long[] steps;

    public static void Main()
    {
        var W = Console.ReadLine().Split().Select(int.Parse).ToArray();
        int N, M;
        N = W[0];
        M = W[1];
        steps = new long[N + 1];
        List<int> a = new List<int>();

        for (int i = 0; i < M; i++)
        {
            int read = int.Parse(Console.ReadLine());
            if (!a.Contains(read)) a.Add(read);
            
        }
        a.Add(-1);

        for (int i = 0; i <= N; i++)
        {
            if (i == 0)
            {
                steps[i] = 1;
            }
            else if (i == 1)
            {
                if (a[0] == i)
                {
                    a.RemoveAt(0);
                    
                    continue;
                }
                else
                {
                    steps[i] = 1;
                }

            }
            else
            {
                if (a[0] == i)
                {
                    
                    steps[i] = 0;
                    a.RemoveAt(0);
                }
                else
                {
                    steps[i] = steps[i - 1] + steps[i - 2];
                    if (steps[i] > 1000000007)
                    {
                        steps[i] %= 1000000007;
                                 //1000000007

                    }
                }

            }

        }
        Console.WriteLine(steps[N]);

    }



}