from fractions import Fraction
import numpy
f = open('3.txt')
N,S,id = map(int,f.readline().split())
A1 = numpy.array(list(map(int,f.readline().split())))
writer = A1[id] + 100 * S
numpy.delete(A1,id)
A = sorted(A1, reverse = True)
P = 1
J = numpy.array([50 * S * (18 + 2 * i) / (8 + 2 * i) for i in range(1,N + 1)])
if A[0] > writer:
	print(0)
else:
	for ai,a in enumerate(A):
		p = 0
		for i in range(N - ai - 1):
			if J[i] + a > writer + 1:
				p = p + 1
			else:
				break
		if p == 0:
			break
		P = P * (1 - p / (N - ai - 1))
		if P == 0:
			break

	print(float(P))