#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)s; i < (int)e; ++i)
#define all(a) (a).begin(), (a).end()

int main() {
    cin.tie(nullptr);
    
    ll N, M, C;
    cin >> N >> M >> C;
    
    vector<ll> A(N), B(M);
    rep(i, 0, N) cin >> A[i];
    rep(i,0 , M) cin >> B[i];
    
    sort(all(A));
    sort(all(B));
    
    ll cnt = 0;
    
    rep(i, 0, N) {
        ll key = C / A[i];
        
        ll ind = upper_bound(all(B), key) - B.begin();
        
        cnt += M - ind;
    }
    
    cout << fixed << setprecision(15) << (double)cnt / (double)(N * M) << '\n';
}