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.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCorder
{
    class Program
    {
        static void Main(string[] args)
        {

            var NM = ReadSplitInt();
            var n = NM[0];
            var m = NM[1];
            var a = ReadSplitIntLines(m);

            var dp = new long[n + 1];
            var instrusion = new bool[n + 1];

            // 1段目への移動方法数
            dp[0] = 1;

            // 侵入負荷の段数設定（trueが侵入負荷）
            for (int i = 0; i < m; i++)
            {
                instrusion[a[i][0]] = true;
            }

            for (int i = 0; i <= n; i++)
            {
                // 1段か2段先へ移動
                for (int j = 1; j <= 2; j++)
                {
                    if (i + j <= n)
                    {
                        // 侵入不可への移動はできない
                        if (instrusion[i + j] != true)
                        {
                            // i番目からi+j番目への移動
                            dp[i + j] += dp[i];
                        }
                    }
                }
            }

            Console.WriteLine(dp[n] % 1000000007);


            Console.ReadKey();
        }



        #region MyRegion

        public static int ReadInt()
        {
            return int.Parse(Console.ReadLine());
        }

        /// <summary>
        /// 空白区切の数値を取得する
        /// </summary>
        /// <returns>The read split int.</returns>
        public static int[] ReadSplitInt()
        {
            return Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        }

        /// <summary>
        /// 空白区切の数値を取得
        /// </summary>
        /// <returns>The read split long.</returns>
        public static long[] ReadSplitLong()
        {
            return Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
        }


        public static int[][] ReadSplitIntLines(int n)
        {
            return Enumerable.Range(0, n).Select(x => ReadSplitInt()).ToArray();
        }
        static long[][] ReadSplitLongLines(int n)
        {
            return Enumerable.Range(0, n).Select(x => ReadSplitLong()).ToArray();
        }

        #endregion

    }
}