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: Runtime Error)

import java.util.*;
 
public class Main{
	public static void main(String[] args){
    	Scanner scan  = new Scanner(System.in);
      	int n = scan.nextInt();	
      	int M = scan.nextInt();	
      	int[] a = new int[M];
      	int ans = 0;
      	ArrayList<Integer> fibo = new ArrayList<>();
      	fibo.add(0);
      	fibo.add(1);
      	//fibo.add(1);
      	for(int  m = 0 ; m < n; m++){
        	fibo.add(fibo.get(m) + fibo.get(m + 1));
        }
      	for(int i = 0; i < M; i++){
        	a[i] = scan.nextInt();	
        }
      	for(int j = 1; j < M; j++){
        	if(Math.abs(a[j] - a[j-1]) == 1){
              	
            	System.out.print("0");
              	System.exit(0);	
            }
        }
      	
      	ans = ans + fibo.get(a[0]);
      	for(int k = 1; k < M; k++){
        	ans = ans + fibo.get(a[k] - a[k - 1]);
        }
      	
      	ans = ans  + fibo.get(n - a[M-1]);
      	if(ans < 1000000007){
        	System.out.print(ans);
        }
      	else{
        	System.out.print(ans % 1000000007);
        }
    }
  	
}