from bisect import bisect_left
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_left(B,C/A[i])
    for j in range(b,M):
        if A[i]*B[j] > C:
            count += (M-j)
            break
print(count/(N*M))