#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    double vl,vr,d,w;
    cin >> vl >> vr >> d >> w;
    printf( "%.8f", w*(d/(vl+vr)) );
    return 0;
}