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
  define('MOD', 1000000007);

  fscanf(STDIN, "%d %d", $N, $M);
  $a = array_pad(array(), $N + 1, 0);
  $a[-1] = 1;
  $a[0] = 0;
  $ans[-1] = 0;
  $ans[0] = 1;

  for ($i = 0; $i < $M; $i++) {
      fscanf(STDIN, "%d", $t);
      $a[$t] = 1;
  }

  for ($i = 1; $i <= $N; $i++) {
    if ($a[$i] === 1) {
        $ans[$i] = 0;
    } else {
        $ans[$i] = $ans[$i - 2] + $ans[$i - 1];
    }
    $ans[$i] %= MOD;
    // printf("%d %d\n", $i, $ans[$i]);
  }

  echo $ans[$N];