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: Runtime Error)

using static System.Console;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Typical_Stairs1
{
    class Program
    {
        static void Main(string[] args)
        {

            string input = ReadLine();
            ulong totalStep = 0;
            ulong totalBrokenStep = 0;
            int count = 0;
            string[] array = input.Split(' ');
            totalStep = ulong.Parse(array[0]);
            totalBrokenStep = ulong.Parse(array[1]);

            ulong[] step = new ulong[totalStep];
            ulong[] BrokenStep = new ulong[totalBrokenStep + 2];

            for (ulong i = 0; i < totalBrokenStep; i++)
            {
                BrokenStep[i] = ulong.Parse(ReadLine());
            }

            step[0] = 1;
            step[1] = 2;

            if (BrokenStep[0] == 1)
            {
                step[0] = 0;
                step[1] = 1;
                count++;
            }
            if (BrokenStep[0] == 2)
            {
                step[1] = 0;
                count++;
            }
            if (BrokenStep[1] == 2)
            {
                step[1] = 0;
                count++;
            }

            for (ulong i = 2; i < totalStep; i++)
            {
                if (i == BrokenStep[count] - 1)
                {
                    step[i] = 0;
                    count++;

                }
                else
                {
                    step[i] = step[i - 1] + step[i - 2];
                }
            }

            //定数！
            const int N  = 1000000007;
            WriteLine(step[totalStep - 1] % N);
            ReadKey();
        }
    }
}