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: Wrong Answer)

<?php
/**
 * ABC129
 * C - Typical Stairs
 */

// $s = trim(fgets(STDIN));
// fscanf(STDIN, "%d %d", $a, $b);
// $n = array_map('intval', explode(' ', trim(fgets(STDIN))));

fscanf(STDIN, "%d %d", $n, $m);

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

$dp[-1] = 0;
$dp[0] = 1;

foreach ($a as $ai) {
    $dp[$ai] = 0;
}
$i_list = array_diff(range(1, $n), $a);

foreach ($i_list as $i) {
    $dp[$i] = ($dp[$i - 1] + $dp[$i - 2]) % 1000000007;
}

echo $dp[$n], PHP_EOL;