from bisect import bisect_right
N,M,C = map(int,input().split())
A = sorted(list(map(int,input().split())))
B = sorted(list(map(int,input().split())))
cnt = 0
for i in range(N):
    a = A[i]
    b = C//a
    ind = bisect_right(B,b)
    cnt += M-ind
print(cnt/(N*M))