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: Accepted)

package main

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

func main() {
	// s[i + 2] = s[i] + s[i+1]
	const mod = 1000000007
	var n, m int
	fmt.Scan(&n, &m)
	s := make([]int64, n+1)
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	var a int
	for i := 0; i < m; i++ {
		sc.Scan()
		a, _ = strconv.Atoi(sc.Text())
		s[a] = -1 // broken floor
	}
	if s[0] >= 0 {
		s[0] = 1
	}
	if s[1] >= 0 {
		s[1] = 1
	}
	for i := 0; i < n-1; i++ {
		if s[i+2] < 0 {
			continue
		}
		if s[i] >= 0 {
			s[i+2] = s[i]
		}
		if s[i+1] >= 0 {
			s[i+2] = (s[i+2] + s[i+1]) % mod
		}
	}
	fmt.Println(s[n])
}