import numpy as np

def findX(l,i,a,c):
    j=i
    for j in range(i,len(l)):
        if(l[j]*a>c):
            break
        if(j==len(l)-1):
            return 0,0
    return len(l)-j,j

[n,m,c]=list(map(int,input().split()))
a=list(map(int, input().split()))
b=list(map(int, input().split()))
b.sort()
a=np.array(a)
b=np.array(b)
a=np.sort(a)[::-1]
count=0
s=n*m
idx=0
for i in range(n):
    po,idx=findX(b,idx,a[i],c)
    count+=po
    if(po==0): 
        break
print(count/s)