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.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int n = sc.nextInt();
		int m = sc.nextInt();
		int []a = new int [m];
		boolean []jdg = new boolean [n+1];
		long []Dp = new long [n+1];
		int p = 1000000007;
		
		for(int i = 0; i < m ; i++) {
			a[i] = sc.nextInt();
			jdg [a[i]] = true;
		}
		Dp[0] = 1;
		
		for(int i = 1 ; i <= n; i++) {
			if(jdg[i] == false) {
			 if(i == 1)
				Dp[i] = Dp[i -1];
			 if(i >= 2) {
				 Dp[i] = (Dp[i-1])%p+(Dp[i-2])%p;
			     Dp[i] = Dp[i]%p;
			}
		}
			if(jdg[i] == true) {
				Dp[i] = 0;
			}
		
		}
		System.out.println(Dp[n]);
	}
		
}