#include<cstdio>
#include <iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

#define mod 1000000007;

using namespace std;
typedef long long ll;
typedef pair<ll, ll> Pr;

ll vl, vr,d,w;

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

	double t = 0,dd;
	while (d > 0) {
		if (d - vr >= 0)
			d -= vr;
		else {
			dd = vr;
			break;
		}
		if (d - vl >= 0)
			d -= vl;
		else {
			dd = vl;
			break;
		}
		t++;
	}
	if (d != 0) {
		t += (double)d / (vr + vl);
	}
	printf("%.10f\n",t*w);
	return 0;
}