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;

public class S
{
    private const bool FINE = true;
    private const bool BROKEN = false;
    public static void Main()
    {



        var input = Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);
        var stairCount = input[0];
        var brokenStairCount = input[1];

        var stairs = new bool[stairCount + 1];
        var patterns = new long[stairCount+1];

        for(long i = 0; i < stairCount; i++){
            stairs[i] = FINE;
            patterns[i] = 0;
        }

        for(long i = 0; i < brokenStairCount; i++){
            stairs[long.Parse(Console.ReadLine()) -1] = BROKEN;
        }

        patterns[0] = stairs[0] ? 1 : 0;
        patterns[1] = stairs[1] ? 1 : 0;
        stairs[stairCount] = BROKEN;

        for(long i = 0; i < stairCount -1; i++){
            if(stairs[i] == FINE){
                if(stairs[i+1] == FINE){
                    patterns[i+1] += patterns[i];
                }
                if(stairs[i+2] == FINE){
                    patterns[i+2] += patterns[i];
                }
            }
        }

        Console.WriteLine(patterns[stairCount-1] % 1000000007);
    }
}