from bisect import *

N, M, C = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
A.sort()
B.sort()
ans = 0
for a in A:
    v = C//a
    ans += M - bisect_right(B, v)
    # print(a, v)
print(ans/N/M)