#include<bits/stdc++.h>
using namespace std;
// #define int long long
typedef long long ll;
typedef pair<int , int> PII;


const int N = 1e5 + 10;

int h , r , da , dw;

long double an(long double x)
{
    long double ans;
    ans = ((dw - da) * x * x + da * h * h) / (2 * ((dw - da) * x + da * h));
    return ans;
}

bool che(long double x)
{
    long double lo = x - 1e-9;
    if(an(lo) < an(x)) return true;
    return false;
}

void solve()
{
    cin >> h >> r >> da >> dw;

    long double l = 0 , r = h;

        // const double eps = 1e-6;   // eps 表示精度，取决于题目对精度的要求
        while (r - l > 1e-10)
        {
            
            long double mid = (l + r) / 2;
            if(che(mid)) r = mid;
            else l = mid;
        }
        
        cout << fixed << setprecision(12) << l << '\n';





}



signed main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int t = 1;
    // cin >> t;
    while(t --)
    {
        solve();
    }

    return 0;
}