n, s, w_id = gets.strip.split(' ').map(&:to_i)
a = gets.strip.split(' ').map(&:to_i)
scores = []
(1..n - 1).each do |i|
	scores << (50 * s + 50 * s / (0.8 + 0.2 * i)).floor	
end
score_writer = a[w_id] + 100 * s
pro = 1.0
(0...n).each do |id|
	next if id == w_id
	count = 0
	scores.each do |score|
		if a[id] + score > score_writer
			count += 1
		else
			break
		end
	end
	pro *= (1.0 - count.to_f / (n - 1))
end

p pro