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
    
def my_round(x,y,a):
    ans=x//y
    rem=x%y
    L=[0]*a
    for i in range(a):
        rem*=10
        temp=(rem//y)
        # print(i,temp)
        L[i]=temp
        rem%=y
    rem*=10
    temp=(rem//y)
    if temp>=5:
        L[-1]+=1
    ans=str(ans)+"."+"".join(map(str,L))
    
    return float(ans)

ans=my_round(ans,N*M,7)

print(ans)