from itertools import combinations
def probability(letter, K):
    final = list(combinations(letter,K))
    count = 0 
    for i in range(len(final)):
        if 'a' in final[i]:
            count += 1 
    prob = count/len(final)
    return prob
    
if __name__ == "__main__":
    N = int(input())
    letter = list(input().split())
    K = int(input())
    result = probability(letter,K)
    print(result)