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 Scala (2.11.7) (judge verdict: Accepted)

object Main extends App {

  val mod = 1000000007

  def mul(a: Int, b: Int): Int = {
    ((a % mod) * (b % mod)) % mod
  }

  def add(a: Int, b: Int): Int = {
    (a + b) % mod
  }


  val Array(n, m) = scala.io.StdIn.readLine.split(" ").map(_.toInt)
  var a = Array.fill(n + 3)(0)

  (1 to m).foreach { _ =>
    val aa = scala.io.StdIn.readLine.toInt
    a(aa) = -99
  }
  a(0) = 1
  //println(a.deep)

  (0 to n).foreach { i =>
    val cnt = a(i)
    if (cnt != -99) {
      if (a(i + 1) != -99) {
        a(i + 1) = add(a(i + 1), cnt)
      }
      if (a(i + 2) != -99) {
        a(i + 2) = add(a(i + 2), cnt)
      }
    }
  }

 // println(a.deep)
  println(a(n))
}