#include <bits/stdc++.h>

using namespace std;
using LL = long long;
using ULL = unsigned long long;
using VI = vector<int>;
using VL = vector<LL>;
using PII = pair<int, int>;
using PLL = pair<LL, LL>;

#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define F first
#define S second
#define FOR(i, a, b) for (int i = (a); i<(b); ++i)
#define RFOR(i, b, a) for (int i = (b)-1; i>=(a); --i)
#define FILL(a, b) memset(a, b, sizeof(a))

const double EPS = 1e-9;

double d, s, r, o;
const double PI = acos(-1);

double g(double OA, double OX, double alpha, double XOA) {
	double XA = sqrt(OX * OX + OA * OA - 2 * OX * OA * cos(XOA));
	double sinOAX = OX * sin(XOA) / XA;
	assert(sinOAX > EPS && sinOAX < 1 - EPS);
	double OAX = asin(sinOAX);
	assert(OAX > alpha - EPS);
	return XA * (OAX - alpha);
}

double f(double b)
{
	double alpha = PI * d / s / 2;
	double OA = s - b;
	double OX = r;
	double cosBeta = OA * sin(alpha) / r;
	if (cosBeta > 1 - EPS) {
		return 0;
	}
	double beta = acos(cosBeta);
	double low = PI / 2 - alpha - beta, high = PI / 2 - alpha + beta;
	
	assert(low > -EPS && high < PI - alpha + EPS);
	
	FOR(it, 0, 747) {
		double m1 = low + (high - low) / 3, m2 = high - (high - low) / 3;
		if (g(OA, OX, alpha, m1) < g(OA, OX, alpha, m2)) {
			low = m1;
		}
		else {
			high = m2;
		}
	}
	return 2 * g(OA, OX, alpha, (low + high) / 2);
}

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	cin >> d >> s >> r >> o;
	
	//cout << f(6.5) << endl;
	//return 0;
	
	double L = r, R = s - r;
	
	if(f(L) > o + EPS){
		cout << "-1.0\n";
		return 0;
	}
	if(f(R) < o - EPS){
		cout << "-2.0\n";
		return 0;
	}	
	
	FOR (i, 0, 474)
	{
		double m = (L + R) / 2;
		double res = f(m);
		if (res < o)
			L = m;
		else
			R = m;
	}
	cout << fixed << setprecision(1) << L - r << '\n';
}