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)

const main = (inputs) => {
  var mod = 1000000007;

  var N = inputs[0].split(" ")[0]-0;
  var M = inputs[0].split(" ")[1]-0;
  var ana = {};
  for(var i=1;i<=M;i++){
    ana[inputs[i]] = true;
    if(i>1 && inputs[i] - inputs[i-1] == 1) {console.log(0);return;}
  }
  var map = {
    0: 1,
    1: (ana[1]?0:1)
  };
  for(var i=2;i<=N;i++){
    if(ana[i]) continue;
    map[i] = (map[i-1]||0) + (map[i-2]||0);
    map[i] = map[i]%mod;
  }
  console.log(map[N]);
}

main(require('fs').readFileSync('/dev/stdin', 'UTF-8').trim().split("\n"));