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.Linq;
using System.Text;
using System.Threading.Tasks;

class Program {
    static void Main(string[] args) {
        //入力
        string s = Console.ReadLine();
        int N = int.Parse(s.Split(' ')[0]);
        int M = int.Parse(s.Split(' ')[1]);

        int[] a = new int[N];

        //変数の割り当て
        for (int i = 0; i < M; i++) {
            a[i] = int.Parse(Console.ReadLine());
        }

        //踏んではいけない階段の間を取る
        int[] diff = new int[M + 1];
        for (int i = 0; i < M + 1; i++) {
            if (i == 0) {
                diff[i] = a[0] + 1;
            } else if (i == M) {
                diff[i] = N - a[i - 1] + 1;
            } else {
                diff[i] = a[i] - a[i - 1];
            }
        }

        //階段の間の段数に応じたインデックスを求める
        int[] index = new int[diff.Max() + 1];
        for (int i = 0; i < diff.Max() + 1; i++) {
            if (i == 0) {
                index[i] = 0;
            } else if (i == 1) {
                index[i] = 1;
            } else {
                index[i] = index[i - 1] + index[i - 2];
            }
        }

        //回答となる数値
        int ans = 1;

        //それぞれの区域について、掛け算を行っていく
        for (int i = 0; i < M + 1; i++){
            ans *= index[diff[i] - 1];
        }

        //出力
        Console.WriteLine(ans %= 1000000007);
    }
}