from bisect import bisect
N,M,C = map(int,input().split())
A = list(map(int,input().split()))
B = list(map(int,input().split()))

A.sort()
B.sort()
count = 0
for i in range(N):
    b = bisect(B,C//A[i])
    if b == M:
        continue
    count += (M-b)
print(count/(N*M))