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.io.*;
import java.util.*;
import static java.lang.System.in;

public class Main {
    static long mod = 1000000000+7;
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        //BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        //String[] buf = reader.readLine().split(" ");
        int n = sc.nextInt(), m = sc.nextInt();
        long[] dp = new long[n+1];
        boolean[] bad = new boolean[n+1];
        for(int i=0;i<m;i++){
            int a = sc.nextInt();
            bad[a] = true;
        }
        dp[0] = 1;
        for(int i=1;i<=n;i++){
            if(bad[i]) continue;
            dp[i] = dp[i-1];
            if(i>1) dp[i] += dp[i-2];
            dp[i] %= mod;
        }
        System.out.println(dp[n]);
    }
}