#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#define mp make_pair
#define pb push_pack
#define LL long long
#define inf 2e9
#define zeros x^(x-1)&x
using namespace std;


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

    cout<<t1*t2 / (t1+t2);

    return 0;
}