/*input
5 5 20 20
*/
#include <bits/stdc++.h>
#define fastIo ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define fi first
#define se second
#define sz size
#define pb push_back
#define mp make_pair
using namespace std;

//#define LOCAL
#ifdef LOCAL
	#define DEBUG(x) do { cout << #x << ": " << x << '\n'; } while (0)
#else
	#define DEBUG(x) 
#endif

const double EPS = 1e-9;
const double PI = 3.141592653589793238462;

int main(){
	fastIo;

	double t1, k1, t2, k2;
	cin >> t1 >> k1 >> t2 >> k2;

	double a = t2 / t1;

	cout << fixed << setprecision(12) << t2 / (a * k1 + k2) << '\n';

	return 0;
}