#include <bits/stdc++.h>
#define NMAX 10005
#define MOD 666013
#define INF 0x3f3f3f3f
#define x first
#define y second
#define ll long long

using namespace std;

int main() {
	double v1,v2,k1,k2,t2,t1,d;

	cin>>t1>>t2>>k1>>k2;

	v1=t2;
	v2=t1;
	d=v1*t1;
	v1*=k1;
	v2*=k2;

	cout<<fixed<<setprecision(5)<<d/v1+d/v2;

	return 0;
}