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;

public class Test
{
	public static void Main()
	{
		// your code goes here
		
		var b = Console.ReadLine().Split().Select(x => int.Parse(x)).ToArray();
		int n = b[0];
		int m = b[1];
		
		bool[] isBroken = new bool[n+10];
		for(int i = 0; i < m; i++){
			int input = int.Parse(Console.ReadLine());
			isBroken[input] = true;
		}
		
		long[] dp = new long[n+10];
		dp[0] = 1;
		
		for(int i = 1; i <= n; i++) {
			if(isBroken[i]){
				dp[i] = 0;
			}else{
				if(i==1){
					dp[i] = dp[i-1];
				}else{
					dp[i] = dp[i-1] + dp[i-2];	
				}
			}
		}
		Console.WriteLine(dp[n]);
	}
}