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.Collections.Generic;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //[summary]C - Typical Stairs
            var line = ReadLine();
            int N = line[0];
            int M = line[1];

            var a = GetBrokenFloors(M);

            var ans = new long[N + 1];
            ans[0] = 1;

            for(int i = 1; i <= N; i++)
            {
                if (!a.ContainsKey(i))
                {
                    if (i == 1)
                    {
                        ans[i] = 1;
                    }
                    else
                    {
                        long sum = ans[i - 1] + ans[i - 2];
                        ans[i] = sum % 1000000007;
                    }
                }
            }

            Console.WriteLine(ans[N]);
        }

        static Dictionary<int, int> GetBrokenFloors(int M)
        {
            var dic = new Dictionary<int, int>();

            for (int i = 0; i < M; i++)
            {
                int a = int.Parse(Console.ReadLine());
                dic.Add(a, 0);
            }

            return dic;
        }

        static List<int> ReadLine()
        {
            var line = Console.ReadLine();
            var arr = line.Split(' ');

            return arr.Select(x => int.Parse(x)).ToList();
        }
    }
}