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: Wrong Answer)

package main

import (
	"fmt"
	"os"
)

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

	inputs := scanNums(M)

	steps := make([]bool, N+1, N+1)

	for i := 0; i <= N; i++ {
		steps[i] = true
	}
	for _, v := range inputs {
		steps[v] = false
		if !steps[v-1] {
			fmt.Println(0)
			os.Exit(0)
		}
	}

	fib := make([]int, N+1, N+1)
	fib[0] = 1
	if steps[1] {
		fib[1] = 1
	} else {
		fib[1] = 0
	}
	for i := 2; i <= N; i++ {
		if steps[i] {
			fib[i] = fib[i-1] + fib[i-2]
		} else {
			fib[i] = 0
		}
		// fmt.Println(i, fib[i])
	}

	res := fib[N] % 1000000007
	fmt.Println(res)

}

func scanNums(len int) (nums []int) {
	var num int
	for i := 0; i < len; i++ {
		fmt.Scan(&num)
		nums = append(nums, num)
	}
	return
}