#include <fstream>
#include <cstring>
#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <iomanip>
#include <algorithm>

#define N 1000000
using namespace std;

int main()
{
	int t1, k1, t2, k2;
	cin >> t1 >> k1 >> t2 >> k2;
	double r = (double)t1 * t2 / (k1 * t2 + k2 * t1);
	cout << r;
	cin >> k1;
	return 0;
}