import bisect

n, m, c = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))
b.sort()

total = 0

for ai in a:
    x = c // ai
    threshold = x + 1
    pos = bisect.bisect_left(b, threshold)
    total += m - pos

probability = total / (n * m)
print("{0:.10f}".format(probability))