from itertools import combinations

n, lc_letters, k = int(input()), input().split(), int(input())  # length of list

combi = list(combinations(lc_letters, k))

num_a_in_comb = 0
for comb in combi:
    if "a" in comb:
        num_a_in_comb += 1
        
print (num_a_in_comb/len(combi))