N,M,C=map(int,input().split())
A=list(map(int,input().split()))
B=list(map(int,input().split()))
A.sort()
B.sort()
p=M-1
numerator=0
for a in A:
    while (p>=0 and B[p]*a>C):
        p-=1
    numerator+=M-1-p
print(numerator/(N*M))