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 Rust (1.15.1) (judge verdict: Runtime Error)

use std::io::stdin;

fn main() {
    let mut input = String::new();
    let _ = stdin().read_line(&mut input).unwrap();
    let n_m = input.trim().split(" ").map(|s| s.parse::<u32>().unwrap()).collect::<Vec<_>>();
    let N = n_m[0];
    let M = n_m[1];

    let a_m: Vec<u32> = (0..M).map(|_| {
        let mut input = String::new();
        let _ = stdin().read_line(&mut input).unwrap();
        input.trim().parse::<u32>().unwrap()
    }).collect();

    let mut step_list = vec![Some(0); N as usize];
    for a in a_m {
        step_list[ a as usize - 1] = None;
    }
    step_list[0] = if step_list[0].is_none() {None} else {Some(1)};
    step_list[1] = if step_list[1].is_none() {None} else {if step_list[0].is_none() {Some(1)} else {Some(2)}};

    for i in 2..N {
        step_list[i as usize] = step_list[i as usize].and_then(|_|{
            if step_list[i as usize -2].is_none() && step_list[i as usize - 1].is_none() {
                None
            } else {
                let acc =  [step_list[i as usize -2], step_list[i as usize - 1]].into_iter().filter_map(|&it| it).fold(0, |sum,ele| (sum + ele) % 1000000007);
                Some(acc)
            }
        })
    }
   println!("{}",if step_list[N as usize - 1].is_some() {step_list[N as usize -1].unwrap()} else {0}); 

}