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 Ruby (2.3.3) (judge verdict: Accepted)

class Array
  def cumsum; s = 0; map{|k| s += k } end
end

n, m = gets.to_s.split.map{|t|t.to_i}
a = Array.new(n){ gets.to_s.to_i }

if n == 1
  puts 1
  exit
end

if n == 2
  puts m == 1 ? 1 : 2
  exit
end

broken = Array.new(n+1){false}
stairs = Array.new(n+1){ 0 }
stairs[0] = stairs[1] = 1

a.each do |k|
  broken[k] = true
end

stairs[1] = 0 if broken[1]
# stairs[2] = 0 if broken[2]

2.upto(n) do |i|
  next if broken[i]
  stairs[i] = stairs[i-1] + stairs[i-2]
end

puts stairs[-1] % (10**9+7)