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: Runtime Error)

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


public class Contest
{
	public static void Main()
	{
		int[] a = Array.ConvertAll(Console.ReadLine().Split(' '), new Converter<string,int>(s => int.Parse(s)));

//		int[] b = Array.ConvertAll(Console.ReadLine().Split(' '), new Converter<string,int>(s => int.Parse(s)));
//		int[] c = Array.ConvertAll(Console.ReadLine().Split(' '), new Converter<string,int>(s => int.Parse(s)));
//		int[] d = Array.ConvertAll(Console.ReadLine().Split(' '), new Converter<string,int>(s => int.Parse(s)));
	
		int[] e = new int[a[1]];
		for(int i=0; i<a[1]; i++){
			e[i] = int.Parse(Console.ReadLine());
		}

		for(int i=0; i<a[1]-1; i++) {
			if(e[i]+1==e[i+1]) {
				Console.WriteLine(0);
				return;
			}
		}
		
		int cc =0;
		ulong es = 1;
		for(int i=0; i<a[1]; i++) {
			if(i==0) { cc = e[i]-1; } else { cc = e[i]-e[i-1]-2; }
			es = ((es * fib((ulong)cc))%1000000007); 
		}
		cc = a[0]-e[a[1]-1]-1;
		es = ((es * fib((ulong)cc))%1000000007); 

		Console.WriteLine(es);
	}
	
	private static ulong fib(ulong n) {
      if (n == 0)
          return 1;
      if (n == 1)
          return 1;
      return fib(n - 1)%1000000007 + fib(n - 2)%1000000007;
	}




}