import bisect

n,m,c = map(int,input().split())
a = list(map(int,input().split()))
b = list(map(int,input().split()))
b = sorted(b)

ans = 0
for x in a:
    k = c/x
    p = bisect.bisect_right(b,k)
    ans += m-p
    #print(k,p)
print(ans/(n*m))