from itertools import combinations

if __name__ == '__main__':
    length,l1,K = int(input()),input().split(),int(input())
    list_combos = list(combinations(l1, K))
    counter1 = 0
    for comb in list_combos:
            if 'a' in comb:
                counter1 += 1
    print(counter1 / len(list_combos))