/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.11.06 21:45:23
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    ll n,m,c;cin >> n >> m >> c;
    vector<ll> 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(b.begin(), b.end());
    ll sum = 0;
    for (int i = 0; i < n; i++) {
        auto t = lower_bound(b.begin(), b.end(), c/a[i]+1);
        sum += b.end()-t;
    }
    printf("%.10f\n",(double)sum/n/m);
    return 0;
}