#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<math.h>
#include<map>
#include<iomanip>
#include<queue>

using namespace std;
using ll = long long;
using ull = unsigned long long;

ll mod = 1e9 + 7;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n , m , c;
    cin >> n >> m >> c;
    vector<int> a(n) , b(m);
    for(int i = 0; i < n; i++){
        cin >> a[i];
    }
    for(int i = 0; i < m; i++){
        cin >> b[i];
    }
    sort(a.begin() , a.end());
    sort(b.begin() , b.end());
    ll cnt = 0;
    int j = m - 1;
    for(int i = 0; i < n; i++){
        while(j >= 0){
            if(a[i] * b[j] <= c){
                break;
            }
            j--;
        }
        cnt += m - 1 - j;
        //cout << cnt << endl;
    }
    cout << (double)cnt / (double)(n*m) << endl;
    return 0;
}