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: Time Limit Exceeded)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func contains(arr []int, el int) bool {
	for _, v := range arr {
		if el == v {
			return true
		}
	}
	return false
}

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Scan()
	nm := strings.Split(sc.Text(), " ")
	n, _ := strconv.Atoi(nm[0])
	m, _ := strconv.Atoi(nm[1])
	bads := []int{}
	for i := 0; i < m; i++ {
		sc.Scan()
		bad, _ := strconv.Atoi(sc.Text())
		bads = append(bads, bad)
	}

	dp := make([]int, n+1)
	dp[0] = 1
	if !contains(bads, 1) {
		dp[1] = 1
	}
	for i := 2; i <= n; i++ {
		if !contains(bads, i-1) {
			dp[i] += dp[i-1]
		}
		if !contains(bads, i-2) {
			dp[i] += dp[i-2]
		}
		dp[i] %= 1000000007
	}
	fmt.Println(dp[len(dp)-1])
}