import bisect

n,m,c=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
b=sorted(b)
cnt=0
for i in range(n):
    pos=bisect.bisect_right(b,c//a[i])
    cnt+=m-pos
print(cnt/(n*m))