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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php 
  list($n, $m) = explode(" ", trim(fgets(STDIN)));

  $mod = 1000000007;

  $now = 0;
  $a = [-1];
  $b = [];
  $dp = [];
  for($i=1; $i<=$m; $i++){
    list($ana) = explode(" ", trim(fgets(STDIN)));
    $a[] = $ana;
    $b[$ana] = 0;
    if($a[$i]-$a[$i-1] == 1){
      echo "0";
      exit;
    }
  }

  if(isset($b[1])){
    $dp = [1, 0];
  }else{
    $dp = [1, 1];
  }
  for($i=2; $i<=$n; $i++){
    if(isset($b[$i])){
      $dp[$i] = 0;
    }
    else{
      $dp[$i] = ($dp[$i-1] + $dp[$i-2]) % $mod;
    }
  }
//  var_dump($dp);
//exit;
  echo $dp[$n];