#include<bits/stdc++.h>
using namespace std;
int main(){
	double a,b,c,d,e,f,g;
	cin>>d>>e>>f>>g;
    a = d*e;
    b = f*g;
    c = (a-b)/(e-g)/1;
    cout<<fixed<<setprecision(2)<<c;
	return 0 ;
}