#include<bits/stdc++.h>
using namespace std;
double x,a,y,b;
int main(){
	cin>>x>>a>>y>>b;
    cout<<fixed<<setprecision(2)<<(x*a-y*b)/(a-b)<<endl;
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
double x,a,y,b;
int main(){
	cin>>x>>a>>y>>b;
    cout<<fixed<<setprecision(2)<<(x*a-y*b)/(a-b)<<endl;
	return 0;
}

# include<bits/stdc++.h>
using namespace std;
int main(){
    double x, a, y, b;
	cin >> x >> a >> y >> b;
    cout << fixed << setprecision(2) << (x * a - y * b) / (a - b) << endl;
	return 0;
}

eee......