import bisect
N,M,C=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
a.sort()
b.sort()
ans=0
def is_ok(arg):
    return arg*i>C
def m_bisect(ng, ok):
    while (abs(ok - ng) > 1):
        mid = (ok + ng) // 2
        if is_ok(mid):
            ok = mid
        else:
            ng = mid
    return ok
for i in a:
    tmp=m_bisect(0,10**9+1) 
    ans+=len(b)-bisect.bisect_left(b,tmp)
print(ans/len(a)/len(b))