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;

class Program
{
    public static void Main()
    {
        var line = Console.ReadLine().Split(' ').Select(p => long.Parse(p)).ToArray();
        long N = line[0];
        long M = line[1];
        long r = 0;
        long r_ = 1;
        long r__ = 0;
        bool k = true;
        bool[] broken = new bool[N + 1];

        for (long i = 0; i < M; i++)
        {
            broken[long.Parse(Console.ReadLine())] = true;
        }

        for (long i = 1; i <= N; i++)
        {
            if (broken[i])
            {
                r = 0;
            }
            else
            {
                r = r_ + r__;
                r = r % 1000000007;
            }
            r__ = r_;
            r_ = r;
        }

        Console.WriteLine(r);
        
    }

}