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.Numerics;
using System.Linq;

namespace c
{
    class Program
    {
        public static readonly long INF = 999999999999;

        static void Main(string[] args)
        {
            var nm = Scanner.IntScan();
            var a = Scanner.IntScan(nm[1]);

            var dp = new long[nm[0] + 10];
            for (var i = 0; i < nm[0]; i++) dp[i] = 0;
            dp[0] = 1;
            var j = 0;
            for(var i = 1; i <= nm[0]; i++)
            {
                if (a[j][0] != i)
                {
                    dp[i] += dp[i - 1];
                    if(i > 1) dp[i] += dp[i - 2];
                }
                else if(a.Length-1 > j) j++;
            }
            Console.WriteLine(dp[nm[0]] % 1000000007);
            
        }
    }

    public class Scanner
    {
        public static string[] Scan() => Console.ReadLine().Split(' ');
        public static int[] IntScan() => Array.ConvertAll(Scan(), int.Parse);
        public static long[] LongScan() => Array.ConvertAll(Scan(), long.Parse);
        public static string[][] Scan(int n) => Enumerable.Repeat(0, n).Select(_ => Scan()).ToArray();
        public static int[][] IntScan(int n) => Enumerable.Repeat(0, n).Select(_ => IntScan()).ToArray();
        public static long[][] LongScan(int n) => Enumerable.Repeat(0, n).Select(_ => LongScan()).ToArray();

    }

    public class Ex
    {
        public static bool Chmin(ref int a, int b) { if (a > b) { a = b; return true; } return false; }
        public static bool Chmin(ref long a, long b) { if (a > b) { a = b; return true; } return false; }
        public static bool Chmax(ref int a, int b) { if (a < b) { a = b; return true; } return false; }
        public static bool Chmax(ref long a, long b) { if (a < b) { a = b; return true; } return false; }
        public static int[] SubArr(int[] arr, int begin, int n) { int[] a = new int[arr.Length]; Array.Copy(arr, begin, a, 0, n); return a; }
    }
}