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 (2.7.1) (judge verdict: Accepted)

n, m = gets.chomp.split(" ").map(&:to_i)
# arr = [*1..n]
# i = 1
 #nozoku = []


# nozoku.each do |j|
#     arr.delete(j)
# end

suuchi = Array.new(n+1,0)

m.times do |i|
    a = gets.chomp.to_i
    #nozoku << a
    suuchi[a] = -1
end
#p nozoku
i = 1
until i-1 == n
    #if nozoku.include?(i)
     #   suuchi[i] = 0
        # puts "UU"
        # p suuchi
    if i >= 3 && suuchi[i] == 0
        # puts "UU"
        # p  suuchi[i-1] 
        # p suuchi[i-2]
        suuchi[i] = [suuchi[i-1],0].max + [suuchi[i-2],0].max
    elsif i >= 3 && suuchi[i] == -1
        suuchi[i] = 0
    elsif i == 1 && suuchi[1] == 0
        suuchi[i] = 1
    elsif i == 1 && suuchi[1] == -1
        suuchi[i] = 0
    elsif i == 2 && suuchi[1] == 1 && suuchi[2] == 0
        suuchi[i] = 2
    elsif i == 2 && suuchi[1] == 0  && suuchi[2] == 0
        suuchi[i] = 1
    elsif i == 2 && suuchi[1] == 0  && suuchi[2] == -1
        suuchi[i] = 0
    end
    
    if suuchi[i] >= 1000000007
        suuchi[i] = suuchi[i] % 1000000007
    end
    #p suuchi
    i += 1
end
#p suuchi.pop % 1000000007
puts suuchi[-1]
# end