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

namespace ABC129C
{
    class Program
    {
        static int[] dp;
        static int Max = (int)Math.Pow(10, 9) + 7;
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split();
            int N = int.Parse(input[0]);
            dp = new int[N + 1];
            for (int i = 0; i < N + 1; i++)
            {
                dp[i] = Max;
            }
            dp[0] = 1;
            dp[1] = 1;
            int M = int.Parse(input[1]);
            int[] a = new int[M];
            for (int i = 0; i < M; i++)
            {
                a[i] = int.Parse(Console.ReadLine());
                dp[a[i]] = 0;
            }
            for (int i = 0; i <= N; i++)
            {
                dp_Sum(i);
            }
            Console.WriteLine(dp_Sum(N));
        }
        static int dp_Sum(int i)
        {
            if (i == 0)
            {
                return 1;
            }
            if (i == 1)
            {
                return 1;
            }
            dp[i]=Math.Min(dp[i],(dp[i - 1]%Max + dp[i - 2]%Max)%Max);
            return dp[i];
        }
    }
}