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: Wrong Answer)

using System;
using System.Text;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
class Program
{
    static void Main(string[] args)
    {
        string[] inputnm = Console.ReadLine().Split(' ');
        int N = int.Parse(inputnm[0]);
        int M = int.Parse(inputnm[1]);
        int[] a = new int[M+1];
        for(int i = 0; i < M; i++)
        {
            a[i] = int.Parse(Console.ReadLine());
        }
        a[M] = N + 1;
        int[] D = new int[M];
        int ans = N;

        if (M == 0) { Console.WriteLine(N); }
        else
        {
            for (int i = 0; i < M; i++)
            {
                D[i] = a[i + 1] - a[i];
                if (D[i] == 1)
                {
                    ans = 0;
                    break;
                }
                if (D[i] == 0)
                {
                    D[i] = 2;
                }

            }
            if (ans == 0)
            {
                Console.WriteLine(ans);
            }
            else
            {
                if (a[0] == 1) { ans = 1; }
                else { ans = a[0] - 1; }
                for (int i = 0; i < M; i++)
                {
                    ans = ans + D[i] - 2;
                }
                Console.WriteLine(ans % 1000000007);
            }

        }
        


    }
}