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: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;

namespace ABC129_1
{
    class C
    {
        static void Main(string[] args)
        {
            var a = Console.ReadLine().Split().Select(long.Parse).ToArray();
            var N = a[0]; var M = a[1];
            var b = Enumerable.Repeat(0, (int)M).Select(_ => long.Parse(Console.ReadLine())).ToArray();
            var mod = 1000000007;

            var res = new long[N + 1];
            res[0] = 1;
            res[1] = 1;
            //Console.WriteLine(Array.BinarySearch(b, 1));

            for (int i = 1; i <= N; i++)
            {
                if (Array.BinarySearch(b, i) >= 0) { res[i] = 0; }
                else if (1 < i) { res[i] = res[i - 1] % mod + res[i - 2] % mod; }
            }
            Console.WriteLine(res[N] % mod);
        }
    }
}