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: Time Limit Exceeded)

import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = Integer.parseInt(sc.next());
		long m = Long.parseLong(sc.next());

		ArrayList<Integer> aArrayList = new ArrayList<>();

		for (int i=0; i<m; i++){
			aArrayList.add(Integer.parseInt(sc.next()));
		}

		ArrayList<Long> dp = new ArrayList<>();
		dp.add(1L);
		if (aArrayList.contains(1L)){
			dp.add(0L);
		}else {
			dp.add(1L);
		}

		for (int i = 2; i <= n; i++) {
			long tmp = 0L;
			if (!aArrayList.contains(i-2)){
				tmp += dp.get(i-2) % 1000000007;
			}
			if (!aArrayList.contains(i-1)){
				tmp += dp.get(i-1) % 1000000007;
			}
			dp.add(tmp);
		}
		System.out.println(dp.get(n) % 1000000007);
	}
}