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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main {
	
	static long MOD  = 1000000007;
	
	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		int n=sc.nextInt(),m=sc.nextInt();
		HashSet<Integer> broken = new HashSet<>();
		
		for(int i =0;i<m;i++)broken.add(sc.nextInt());
		
		long[] dp = new long[n+1];
		
		
		dp[0]=1;
		if(broken.contains(1))dp[1]=0;
		else dp[1]=1;
		
		for(int i=2;i<n+1;i++) {
			if(broken.contains(i)) {
				dp[i]=0;
			}else{
				dp[i]=dp[i-2]%MOD +dp[i-1]%MOD;
			}
		}
		
		//for(int i=0;i<n+1;i++)System.out.println(dp[i]);
		long ans = dp[n]%MOD;
		System.out.println(ans);
		
		
	}

}