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: Accepted)

using System;
using System.Linq;
using System.Collections.Generic;
using System.Text;
using static System.Console;
using static System.Math;

class Program
{
    static void Main(string[] args)
    {
        int[] NM = ReadLineParseIntArray();
        int N = NM[0];
        int M = NM[1];
        long ans = 1;
        long[] stairs = new long[N + 1];
        bool[] canNotMove = new bool[N + 1];
        for (int i = 0; i < M; i++)
            canNotMove[ReadLineParseInt()] = true;

        stairs[0] = 1;
        stairs[1] = canNotMove[1] ? 0 : 1;
        for(int i = 2; i <= N; i++)
        {
            if (canNotMove[i]) continue;
            long v = 0;
            v += canNotMove[i - 1] ? 0 : stairs[i - 1];
            v += canNotMove[i - 2] ? 0 : stairs[i - 2];
            stairs[i] = v % 1000000007;
        }
        WriteLine(stairs[N]);
    }
    static int[] ReadLineParseIntArray() => ReadLine().Split(' ').Select(int.Parse).ToArray();
    static int ReadLineParseInt() => int.Parse(ReadLine());
}