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