import itertools

n = int(input())

letters = input().split()

k = int(input())

count = letters.count('a')

total_combinations = len(list(itertools.combinations(range(n),k)))

favorable_combinations = 0

for comb in itertools.combinations(range(n), k):

if any(letters[i] == 'a' for i in comb):
    favorable_combinations += 1

probability = favorable_combinations/total_combinations print(f"{probability:.4f}")