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 System;
using System.Collections.Generic;
using System.Linq;

public class ABC0129_A3v1 {
    private static void Main(string[] args) {
        var numStr = Console.ReadLine().Trim().Split();
        var numOfSteps = int.Parse(numStr[0]);
        var numOfOffLimitsPositions = int.Parse(numStr[1]);

        var offLimitsPositions = new int[numOfOffLimitsPositions];
        for (var index = 0; index < numOfOffLimitsPositions; index++) {
            var input = Console.ReadLine().Trim();
            offLimitsPositions[index] = int.Parse(input);
        }

        Compute(numOfSteps, offLimitsPositions);
    }

    private static void Compute(int numOfSteps, int[] offLimitsPositions) {
        var diffOfOffLimits = offLimitsPositions[0];
        long totalOfWayOfGoUp = CalcNumOfWayOfGoUp(diffOfOffLimits);

        for (var index = 1; index < offLimitsPositions.Length; index++) {
            diffOfOffLimits = offLimitsPositions[index] - offLimitsPositions[index - 1];
            totalOfWayOfGoUp = CalcNewTotalOfWayOfGoUp(totalOfWayOfGoUp, diffOfOffLimits);
        }

        diffOfOffLimits = numOfSteps - offLimitsPositions[offLimitsPositions.Length - 1];
        totalOfWayOfGoUp = totalOfWayOfGoUp * CalcNumOfWayOfGoUp(diffOfOffLimits) % 1000000007;

        OutputAnswer(totalOfWayOfGoUp.ToString());
    }

    private static long CalcNewTotalOfWayOfGoUp(long totalOfWayOfGoUp, int diffOfOffLimits) {
        if (diffOfOffLimits == 1) {
            return 0;
        }

        return totalOfWayOfGoUp * CalcNumOfWayOfGoUp(diffOfOffLimits) % 1000000007;
    }

    private static int CalcNumOfWayOfGoUp(int diffOfOffLimits) {
        if (diffOfOffLimits <= 1)
            return 1;

        var sequence = new int[diffOfOffLimits];
        sequence[0] = sequence[1] = 1;

        for (var index = 2; index < diffOfOffLimits; index++) {
            sequence[index] = sequence[index - 1] + sequence[index - 2];
        }

        return sequence[diffOfOffLimits - 1];
    }

    private static void OutputAnswer(string text) => Console.WriteLine(text);
}