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 Go (1.6) (judge verdict: Runtime Error)

package main

import "fmt"

var mod int = 1000000007
var N, M int
var a []int
var memo []int

func main() {
	fmt.Scan(&N, &M)

	a = make([]int, N+1)
	for i := 0; i < M; i++ {
		var t int
		fmt.Scan(&t)
		a[t] = 1
	}
	memo = make([]int, N+1)
	fmt.Println(dfs(N))
}

func dfs(n int) int {
	if n < 0 || a[n] == 1 {
		return 0
	}
	if memo[n] != 0 {
		return memo[n]
	}
	if n == 0 {
		return 1
	}

	x := map[bool]int{true: memo[n-1], false: dfs(n - 1)}[memo[n-1] != 0]
	y := map[bool]int{true: memo[n-2], false: dfs(n - 2)}[memo[n-2] != 0]

	memo[n] = (x + y) % mod
	return memo[n]
}