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

def nibutan_max(a, L, R, x):
    while L <= R:
        M = (L + R) // 2
        if a[M] <= x:
            L = M + 1
        elif x < a[M]:
            R = M - 1
    return R

ans = 0
for x in a:
    y = nibutan_max(b, 0, len(b) - 1, c / x)
    if y == 0 and x * b[0] > c:
        ans += len(b)
    elif y == 0:
        ans += len(b) - 1
    else:
        ans += len(b) - y - 1

print(ans/ (n * m))