#include <bits/stdc++.h>

using namespace std;

#define INF 2000000000
#define MOD 1000000007
typedef long long ll;
typedef pair<int, int> P;


int main()
{
	double vl, vr, d, w;
	cin >> vl >> vr >> d >> w;

	double ret = (w * d)/(vr+vl);

	printf("%f\n",ret);

}