import sys
input = sys.stdin.readline
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()
c = 0

for ai in a:
    c += M-bisect_right(b, C//ai)

print(c/N/M)