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

namespace AtCoder_Grand_Contest_034 {
	class Program {
		static void Main(string[] args) {
			var input = Helper<int>.ReadLine();
			var n = input[0];
			var m = input[1];

			var aList = Helper<int>.Read(m);

			var currentPosition = 0;
			var stepLog = new List<int> { 0 };
			while (currentPosition != n) {
				if (!aList.Contains(currentPosition + 1)) {
					stepLog.Add(currentPosition + 1);
					currentPosition += 1;
				} else if (!aList.Contains(currentPosition + 2)) {
					stepLog.Add(currentPosition + 2);
					currentPosition += 2;
				} else {
					Console.WriteLine(0);
					return;
				}
			}

			var skipableCount = 0;
			for (int index = 0; index < stepLog.Count - 2; index++) {
				if (stepLog[index] + 2 == stepLog[index + 2]) {
					skipableCount++;
				}
			}

			Console.WriteLine(Math.Pow(2, skipableCount) % 1000000007);
		}
	}

	public static class Helper<T> {
		public static T[] Read(int count) {
			var result = new T[count];
			for (int index = 0; index < count; index++) {
				result[index] = (T)TypeDescriptor.GetConverter(typeof(T)).ConvertFromString(Console.ReadLine());
			}
			return result;
		}

		public static T[] ReadLine() {
			var input = Console.ReadLine();
			var inputs = input.Split(' ');
			var result = new T[inputs.Count()];
			for (int index = 0; index < inputs.Count(); index++) {
				result[index] = (T)TypeDescriptor.GetConverter(typeof(T)).ConvertFromString(inputs[index]);
			}
			return result;
		}
	}
}