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;
using static System.Console;
using System.Text;
 
namespace AtCoder_Beginner_Contest_129
{
    class Program
    {
        static public int[] Sarray() { return ReadLine().Split().Select(int.Parse).ToArray(); }
        static public List<int> Slist() { return ReadLine().Split().Select(int.Parse).ToList(); }
 
        static void Main(string[] args)
        {
            var NM = Sarray();
            var N = NM[0];
            var M = NM[1];
            var am = new int[N];
            for (var i = 0; i < M; ++i)
            {
                am[Sarray()[0] - 1] = 1;
            }
            var num = new long[N];
            if (N == 1)//1の場合
            {
                WriteLine(1);
                return;
            }
            if (0 == am[0]) num[0] = 1;
            if (0 == am[1]) num[1] = 1 + num[0];
            if (num[0] == 0 & num[1] == 0)
            {
                WriteLine(0);
                return;
            }
            for (var i = 2; i < N; ++i)
            {
                if (0 != am[i])
                { continue; }
                num[i] = (num[i - 1] + num[i - 2])%1000000007;
            }
            WriteLine(num[N - 1]);
        }
    }
}