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.Linq;

namespace abc129
{
    class C2
    {

        const int MOD = 1000000007;
        static public bool[] isHolls;

        static void Main(string[] args)
        {
            var input = Input.IntArray();
            var N = input[0];
            var M = input[1];

            isHolls = new bool[N + 1];

            for (int i = 0; i < M; i++)
            {
                isHolls[Input.Int()] = true;
            }

            var dp = new long[N + 1];

            for (int now = 0; now < N; now++)
            {
                for (int next = now + 1; next < Math.Min(N, now + 2); next++)
                {
                    dp[next] += dp[now];
                    dp[next] %= MOD;
                }
            }

            Console.WriteLine(dp[N]);

        }


        /** Input Class */
        internal static class Input
        {
            /** input int */
            public static int Int() => int.Parse(Console.ReadLine());

            /** input str */
            public static string Str() => Console.ReadLine();

            /** input array of int */
            public static int[] IntArray() => Console.ReadLine().Split(' ').Select(int.Parse).ToArray();

            /** input array of string */
            public static string[] StrArray() => Input.Str().Split(' ');

            /** input array of char */
            public static char[] charArray() => Input.Str().ToCharArray();
        }

    }
}