from collections import Counter
from bisect import bisect_right
n,m,c = map(int,input().split())
a = list(map(int,input().split()))
b = list(map(int,input().split()))

b.sort()
ans = 0
for i in range(n):
    elem = c // a[i]
    ind = bisect_right(b,elem)
    ans += m - ind
print(ans / (n * m))