#include <iostream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <queue>

#include <map>
#include <set>
#include <string>
#include <algorithm>
#include <functional>
using namespace std;
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define INF 1<<29
#define ALEN(ARR) (sizeof(ARR) / sizeof((ARR)[0]))
#define MP make_pair
#define mp make_pair
#define pb push_back
#define PB push_back
#define _DEBUG(x) cout<<#x<<": "<<x<<endl
#define _DDEBUG(x,y) cout<<#x<<": "<<x<<", "<<#y<<": "<<y<<endl
#define ll long long
#define ull unsigned long long
#define MOD 1000000007

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout.precision(16);

    double vl, vr, d, w;
    cin >> vl >> vr >> d >> w;
#if DEBUG
    cout << "** RESULT **" << endl; // debug
#endif
    cout << (w * d / (vl + vr)) << endl;

    return 0;
}