Horse-Land

#include<bits/stdc++.h>
#include <iostream>
#include <cmath>
#include<string>
#include<cstdbool>
using namespace std;
void solve ();
float h, l, r, k;
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() {

    float mn = FLT_MAX;
    float mni;
    for (float i = 0; i <= r; i++) {
        if(sqrt(h * h + i * i) + k * sqrt(l * l + (r - i) * (r - i))<mn){
            mn= sqrt(h * h + i * i) + k * sqrt(l * l + (r - i) * (r - i));
            mni=i;
        }
    }
    //cout << mni << endl;
    float mn1= FLT_MAX, mni1;
    for (float i = mni-1; i <= mni+1; i+=0.01)
    {
        if(sqrt(h * h + i * i) + k * sqrt(l * l + (r - i) * (r - i))<mn1){
            mn1= sqrt(h * h + i * i) + k * sqrt(l * l + (r - i) * (r - i));
            mni1=i;
        }
    }
    float i=mni1;
    cout<<sqrt(h * h + i * i) +  sqrt(l * l + (r - i) * (r - i))<<endl;

}