N,S,id = map(int,input().split())
A1 = list(map(int,input().split()))
writer = A1[id] + 100 * S
A1.pop(id)
A = sorted(A1, reverse = True)
P = 1
J = [50 * S + (500 * S) // (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:
				p = p + 1
			else:
				break
		if p == 0:
			break
		P = P * (1 - p / (N - ai - 1))
		if P == 0:
			break
	print(P)