from itertools import combinations
n = int(input())
eng_list = input().split()  
k = int(input())
index_a = [i+1 for i,j in enumerate(eng_list)if j=='a']
comb = list(combinations(range(1,len(eng_list)+1), k))
count = 0
for j in comb:
    if any(i in j for i in index_a):
        count+=1
prob = count/len(comb)
print(round(prob,11))