#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    int N,M; cin >> N >> M;
    long double C; cin >> C;
    vector<long double> a(N),b(M);
    rep(i,N) cin >> a[i];
    rep(i,M) cin >> b[i];
    sort(b.begin(), b.end());
    
    ll cnt = 0;
    rep(i,N){
        cnt += b.end() - upper_bound(b.begin(), b.end(), C / a[i]);
    }
    cout.precision(17);
    cout << (double)cnt / (double)(N * M) << '\n';
}