#include <bits/stdc++.h>
using namespace std;

int main() {
    long long N,M;
    long long C;
    cin >> N >> M >> C;
    vector<long long>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());
    long long cnt = 0;
    for(int i = 0; i < N; i++) {
        long long tmp = C/a[i]+1;
        int it = lower_bound(b.begin(),b.end(),tmp)-b.begin();
        cnt += M-it;
    }
    cout << fixed << setprecision(18) << (long double)(cnt)/(N*M) << endl;
}