/* -*- coding: utf-8 -*-
 *
 * 1280.cc:  No.1280 Beyond C - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100000;
const int MAX_M = 100000;

/* typedef */

typedef long long ll;

/* global variables */

ll as[MAX_N], bs[MAX_M];

/* subroutines */

/* main */

int main() {
  int n, m;
  ll c;
  scanf("%d%d%lld", &n, &m, &c);

  for (int i = 0; i < n; i++) scanf("%lld", as + i);
  for (int i = 0; i < m; i++) scanf("%lld", bs + i);
  sort(bs, bs + m);

  ll sum = 0;
  for (int i = 0; i < n; i++) {
    int k = upper_bound(bs, bs + m, c / as[i]) - bs;
    sum += m - k;
  }

  printf("%.10lf\n", (double)sum / ((ll)n * m));
  return 0;
}