#include <iostream>
#include <iomanip>
#include <vector>
#include <cassert>
#include <algorithm>
#define rep(i,n) for(int i=0; i<(int)(n); i++)

using namespace std;
using LL = long long;
const int Max_N = 1e5;
const LL Max_C = 1e18;
const LL Max_A = 1e9;

int main(){
  LL N, M, C;
  cin >> N >> M >> C;
  assert(1 <= N and N <= Max_N);
  assert(1 <= M and M <= Max_N);
  assert(1 <= C and C <= Max_C);
  vector<LL> a(N), b(M);
  rep(i,N) cin >> a[i];
  rep(i,N) assert(1 <= a[i] and a[i] <= Max_A);
  rep(i,M) cin >> b[i];
  rep(i,M) assert(1 <= b[i] and b[i] <= Max_A);
  sort(b.begin(),b.end());
  double ans = 0;
  rep(i,N){
  	int left = -1, right = M;
  	int mid = (left + right) / 2;
  	while(right - left > 1){
  		if(a[i] * b[mid] > C) right = mid;
  		else left = mid;
  		mid = (left + right) / 2;
  	}
  	ans += M - right;
  }
  ans /= N * M;
  cout << setprecision(15) << ans << endl;

  return 0;
}