from bisect import bisect_left, bisect_right
n, m, c = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))
a.sort()
b.sort()
ALL = n*m
good = 0
for v in a:
    good += m - bisect_right(b, c//v)
print(good/ALL)