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 JavaScript (node.js v5.12) (judge verdict: Accepted)

'use strict'
function main(input) {
    const tmp = Array.from(input.split('\n'));
    const NM = tmp.shift().split(' ').map((elem) => parseInt(elem), 10);
    const N = NM[0];
    const M = NM[1];

    const mod = 1000000007;
    const A = tmp.map((elem) => parseInt(elem, 10));
    let dp = Array(N + 1).fill(0);
    for (let a of A) {
        dp[a] = -1;
    }
    dp[0] = 1;

    for (let i = 0; i < N; i++) {
        if (dp[i] === -1) {
            continue;
        }
        if (dp[i + 1] !== -1) {
            dp[i + 1] += dp[i];
            dp[i + 1] %= mod;
        }
        if (dp[i + 2] !== -1) {
            dp[i + 2] += dp[i];
            dp[i + 2] %= mod;
        }


    }
    console.log(dp[N]);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));