#include <bits/stdc++.h>

using namespace std;

double t1,t2,k1,k2,v1,v2,ans;
int main(){
    cin>>t1>>k1>>t2>>k2;
    v2=t1;
    v1=t2;
    t1/=k1;
    t2/=k2;
    ans=(t1*k1*v1/(k1*v1+k2*v2));
    cout<<ans;
}