#include <bits/stdc++.h>
typedef  long long ll;
#define INF 1000000000
using namespace std;
#define N 1000000
#define MOD 1000000007
int main(void){
    double vr,vl,d,w,t,ans;
    cin>>vl>>vr>>d>>w;
    t=(d/(vl+vr));
    ans=t*w;
    printf("%lf",ans);

}