from itertools import combinations

fruitless = int(input())
letters = input().split()
indcies = int(input())

combos = sum(1 for i in combinations(letters, indcies))
combo_with_A = sum(1 for c in combinations(letters, indcies) if "a" in c)
probability = combo_with_A / combos

print(f"{probability:.3f}")