from itertools import combinations

Reading input

n = int(input())

letters = input().split()

k = int(input())

Finding the indices where the letter 'a' occurs

a_indices = [i for i, letter in enumerate(letters) if letter == 'a']

Generating all combinations of k indices from the list

total_combinations = list(combinations(range(n), k))

Counting the combinations that contain at least one 'a' index

favorable_combinations = [ combo for combo in total_combinations if any(i in a_indices for i in combo) ]

Calculating the probability

probability = len(favorable_combinations) / len(total_combinations)

Printing the result with 4 decimal places print(f"{probability:.4f}")