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;
class Program
    {
        static void Main(string[] args)
        {
            //まずは数字を読む

            string[] input = Console.ReadLine().Split(' ');
            int n = int.Parse(input[0]);
            int m = int.Parse(input[1]);

            int[] fallstair = new int[m + 1];
            ulong[] stair = new ulong[n + 1];

            //とりあえず全部読みこむ
            for (int i = 0; i <m;i++)
                fallstair[i] = int.Parse(Console.ReadLine());

            //ない階段を数える
            int count = 0;

            //結果出力
            int output = 0;

            //初期条件設定
            if (m >= 2)
            {
                if (fallstair[count] == 1 && fallstair[count + 1] == 2)
                {
                    output = 0;
                    goto end;
                }
                else if (fallstair[count] == 1 && fallstair[count + 1] != 2)
                {
                    stair[0] = 0;
                    stair[1] = 1;
                    count++;
                }
                else if (fallstair[count] == 2)
                {
                    stair[0] = 1;
                    stair[1] = 0;
                    count++;
                }
                else
                {
                    stair[0] = 1;
                    stair[1] = 2;

                }
            }
            else if(m == 1)
            {
                if (fallstair[count] == 1)
                {
                    stair[0] = 0;
                    stair[1] = 1;
                    count++;
                }
                else if (fallstair[count] == 2)
                {
                    stair[0] = 1;
                    stair[1] = 0;
                    count++;
                }
                else
                {
                    stair[0] = 1;
                    stair[1] = 2;

                }
            }

            for (int i = 2; i < n; i++)
            {
                int fall = fallstair[count];

                if (fall == i + 1)
                {
                    stair[i] = 0;
                    count++;
                }
                else
                    stair[i] = stair[i - 1] + stair[i - 2];

                //ダメなときは出す
                if (stair[i] == 0 && stair[i - 1] == 0)
                {
                    output = 0;
                    goto end;
                }

            }

            output = (int)(stair[n - 1] % 1000000007);

            //普通に終われば普通に出す


            
            //最初からダメなら求めない
            end:

            ;

            Console.WriteLine(output);


        }
    }