#include <bits/stdc++.h>
#define rep(i, n) for(int (i) = 0; (i) < (n); (i)++)
#define repe(i, n) for(i = 0; i <= (n); i++)
#define repr(i, n) for(int (i) = (n); (i) > 0; --(i))
#define repre(i, n) for(int (i) = (n); (i) >= 0; --(i))
using namespace std;
using ll = long long;
using ull = unsigned long long;
using P = pair<int,int>;
using Graph = vector<vector<int>>;

const int INF = INT32_MAX;

const ll MOD = 1e9+7;

signed main()
{
    ll n, m, c;
    cin >> n >> m >> c;
    vector<ll> a(n,0), b(m,0);
    rep(i,n)cin >> a[i];
    rep(i,m)cin >> b[i];
    sort(a.begin(), a.end());
    sort(b.begin(), b.end());
    ll ans = 0;
    rep(i,n){
        ll low = c / a[i];
        low++;
        auto l = lower_bound(b.begin(), b.end(), low);
        auto index = distance(b.begin(), l);
        ans += (m - index);
    }
    double as = ans / double(n*m);
    cout << as << endl;
    return 0;

}