n,m,c=map(int,input().split())
a=sorted(list(map(int,input().split())),reverse=True)
b=sorted(list(map(int,input().split())))
now=0
ans=0
for i in range(n):
    while now<m and a[i]*b[now]<=c:
        now+=1

    ans+=m-now

print(ans/(n*m))