#include "bits/stdc++.h"
#define REP(i,n,N) for(ll i=(n); i<(N); i++)
#define RREP(i,n,N) for(ll i=(N-1); i>=n; i--)
#define CK(n,a,b) ((a)<=(n)&&(n)<(b))
#define ALL(v) (v).begin(),(v).end()
#define p(s) cout<<(s)<<endl
#define p2(a,b) cout<<(a)<<" "<<(b)<<endl
typedef long long ll;
using namespace std;
const ll inf=1e18;


int main(){
    double v1,v2,d,w;
    cin>>v1>>v2>>d>>w;
    double time = d/(v1+v2);
    double ans=w*time;
    printf("%.7lf\n", ans);

    return 0;
}