mod = 1000000007
eps = 10**-9


def main():
    import sys
    from bisect import bisect_right
    input = sys.stdin.readline

    N, M, C = map(int, input().split())
    A = list(map(int, input().split()))
    B = list(map(int, input().split()))
    B.sort()

    cnt = 0
    for a in A:
        cnt += M - bisect_right(B, C // a)
    print(cnt / (N * M))


if __name__ == '__main__':
    main()