#include<iostream>
#include<stdio.h>
using namespace std;

#pragma warning(push)
#pragma warning(disable: 4996)

#define NMAX 100005

int t1, t2, k1, k2;
double v1, v2;


int main()
{
	//freopen("mindcoding.in", "r", stdin);
	//freopen("mindcoding.out", "w", stdout);
	
	cin >> t1 >> k1 >> t2 >> k2;

	v1 = (double)t1 / k1;
	v2 = (double)t2 / k2;

	cout << (double)1 / (v1 + v2);

	return 0;
}

#pragma warning(pop)