#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL_
void debug_out() {cerr << endl;}
template<typename Head,typename... Tail> void debug_out(Head H,Tail... T){cerr << ' ' << H; debug_out(T...);}
#define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:",debug_out(__VA_ARGS__)
#define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl;
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
#define rep(i,n) for (int i = 0; i < (int)(n); i ++)
#define irep(i,n) for (int i = (int)(n) - 1;i >= 0;--i)
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;// = 998244353;
constexpr double EPS = 1e-4;
constexpr double PI = 3.14159265358979;

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

    int N,M;
    ll C; cin >> N >> M >> C;
    vector<ll> A(N),B(M);
    rep(i,N) cin >> A[i];
    rep(i,M) cin >> B[i];

    sort(B.begin(),B.end());
    ll ans = 0;
    rep(i,N) {
        ans += M - (upper_bound(B.begin(),B.end(),C/A[i]) - B.begin());
    }
    double prob = (double)ans/N/M;
    cout << prob << '\n';
    return 0;
}