#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;

#define MOD 1000000007

int main() {
  int n, m;
  cin >> n >> m;
  ll c;
  cin >> c;
  vector<ll> a(n), b(m + 2);
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  for (int i = 1; i <= m; ++i) {
    cin >> b[i];
  }
  b[0] = 0, b[m + 1] = 1e18 + 1;
  ll win = 0;
  sort(b.begin(), b.end());
  for (int i = 0; i < n; ++i) {
    int ok = n + 1, ng = 0;
    while (1 < abs(ok - ng)) {
      int mid = (ok + ng) / 2;
      if (c < a[i] * b[mid]) {
        ok = mid;
      } else {
        ng = mid;
      }
    }
    win += n - ok + 1;
  }
  printf("%.15f\n", (double)(win) / (n * m));
  return 0;
}