#include <iostream>
#include <fstream>
#include <cstring>
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <vector>
#include <stack>
#include <list>	
#include <algorithm>
#include <limits.h>

using namespace std;

int main(){;
	int t1, k1, t2, k2;
	cin >> t1 >> k1 >> t2 >> k2;

	double res = (double)t1 * t2 /(k1 * t2 + k2 * t1);

	cout << res << endl;

}