#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
double N,t1,k1,t2,k2;
int main(){
    ios_base::sync_with_stdio(0);cin.tie(0);
    cin >> t1 >> k1 >> t2 >> k2;
    cout << fixed << setprecision(10) << t1/(k1+k2*(t1/t2));
}