#include <iostream>
#include <iomanip>
using namespace std;
int main(){
    int a,b,c,d;cin>>a>>b>>c>>d;
    cout << fixed << setprecision(10); 
    if(b<d)cout << (double)b+(double)abs(d-b)*a/(a+c) << endl;
    else cout << (double)d+(double)abs(d-b)*c/(a+c) << endl;
    
}