N = int(input())
X = list(map(int,input().split()))
sums= 0
res = 0
p = [0]*51
maxx = max(X)
for i in range(1,maxx+1):
sums += p[i-1]
tmp = 1
for x in X:
if i<x:
tmp *= float(i)/float(x)
p[i] = tmp-sums
res += p[i]*float(i)
print('%.2f'%res) 第三题代码,凭记忆复现的,不知道有没有错