#include <bits/stdc++.h>

using namespace std;

int main(int argc, char** argv)
{
	double ax, ay; cin >> ax >> ay;
	double bx, by; cin >> bx >> by;
	double yp = by - (((by-ay)/(bx+ax))*bx);
	cout << setprecision(10) << yp << endl;
	return 0;
}