import sys
input=sys.stdin.readline
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))


N,M,C=MI()
A=LI()
B=LI()
A.sort()
B.sort()


import bisect

ans=0
for i in range(N):
    a=A[i]
    t=C/a
    num=bisect.bisect_right(B,t)
    ans+=M-num
    
print('{:.10f}'.format(ans/(N*M)))