#include<iostream>
using namespace std;
int main(){
	double xa, ya;
	double xb,yb;
	double A, B;
	cin >> xa >> ya;
	cin >> xb >> yb;
	A = (ya - yb) / (xa + xb);
	B = ya - A*xa;
	cout << B <<  endl;
	return 0;
}