House Land

#include<bits/stdc++.h>
#include <iostream>
#include <cmath>
#include<string>
#include<cstdbool>
using namespace std;
void solve ();
double h, l, r, k;
double t(double h, double l, double r, double k );
double le (double h, double l, double r, double k);
double ts(double z, double r);
int main()
{
    int t = 1;
    //cin >> t; //x=t;
    while (cin >> h >> l >> r >> k) {
        if (h || l || r || k)
            solve();
    }
    return 0;
}

void solve() {

    double ans = ts (0, r);
    cout << fixed << setprecision(2) << ans  << endl;

}

double t(double i ) {
    return (sqrt(h * h + i * i) + k * sqrt(l * l + (r - i) * (r - i)));
}
double le (double i) {
    return (double)sqrt(h * h + i * i) + (double) sqrt(l * l + (r - i) * (r - i));
}
double ts(double z, double r) {
    double low = z;
    double high = r;
    double mid1 = low + (r - z) / 3.0;
    double mid2 = high - (r - z) / 3.0;
    double tmid2 = t(mid2);
    double tmid1 = t(mid1);
    if (high -low <= 0.0000001) {
        //cerr << (mid2);
        return le (mid2);
    }
    else if (tmid2 > tmid1) {
        return ts( low, mid2);
    }
    else if (tmid2 < tmid1) {
        return ts(mid1, high);
    }


}