#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

ll n, m, c;
ll a[100000], b[100000];

void input(){
    cin >> n >> m >> c;
    for(int i = 0; i < n; i++) cin >> a[i];
    for(int i = 0; i < m; i++) cin >> b[i];
    sort(a, a+n);
    sort(b, b+m);
}

void solve(){
    ll total = n*m;
    ll cnt = 0;
    for(int i = 0; i < n; i++){
        ll x = c/a[i]+1;
        auto p = lower_bound(b, b+m, x);
        cnt += (b+m)-p;
    }
    cout << (double)cnt/(double)total << endl;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    input();
    solve();
}