///Etfsh5t nseet el rank
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")
#include "bits/stdc++.h"
using namespace std;

#define pb push_back
#define F first
#define S second
#define f(i, a, b) for (int i = a; i < b; i++)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define sz(x) (int)(x).size()
#define mp(x, y) make_pair(x, y)
#define popCnt(x) (__builtin_popcountll(x))
// #define int ll

using ll = long long;
using ull = unsigned long long;
using uint = uint32_t;
using ii = pair<int, int>;

const int N = 3e5 + 5, A = 12, LG = 18, MOD = (119 << 23) + 1;
const long double PI = acos(-1);
const long double EPS = 1e-9;
const int INF = 1e18;
long double h, r, d_air, d_water;
long double calc(long double md)
{
    long double h_water = md;
    long double h_air = h - h_water;
    long double center_water = h_water / 2.0;
    long double center_air = h - h_air / 2.0;
    long double avg_center = (center_air * h_air * d_air + center_water * d_water * h_water) / (d_air * h_air + d_water * h_water);
    return avg_center;
}
void doWork()
{

    cin >> h >> r >> d_air >> d_water;
    long double lo = 0.0, hi = h;
    for (int k = 0; k < 300; k++)
    {
        long double md1 = lo + (hi - lo) / 3;
        long double md2 = hi - (hi - lo) / 3;
        if (calc(md1) < calc(md2))
            hi = md2;
        else
            lo = md1;
    }
    cout << fixed << setprecision(10) << lo << endl;
}
int32_t main()
{
#ifdef ONLINE_JUDGE
    ios_base::sync_with_stdio(0);
    cin.tie(0);
#endif // ONLINE_JUDGE
    int t = 1;
    // cin >> t;
    while (t--)
    {
        doWork();
    }
    return 0;
}