#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))
#define all(x) (x).begin(),(x).end()
#define inf 1e18

using namespace std;

typedef long long llint;
typedef pair<llint, llint> P;

llint n, m, c;
llint a[100005], b[100005];

int main(void)
{
	//ios::sync_with_stdio(0);
	//cin.tie(0);
	
	cin >> n >> m >> c;
	rep(i, 1, n) cin >> a[i];
	rep(i, 1, m) cin >> b[i];
	sort(b+1, b+m+1);
	
	llint ans = 0;
	rep(i, 1, n){
		llint p = upper_bound(b+1, b+m+1, c/a[i]) - b;
		ans += m - p + 1;
	}
	printf("%.11f\n", (double)ans/(n*m));
	
	return 0;
}