#include <iostream>
#include <cstdio>
#include <cassert>
using namespace std;
int main(){
	int X_a,X_b;
	int Y_a,Y_b;
	cin >> X_a >> Y_a;
	cin >> X_b >> Y_b;

	assert(0 < X_a && X_a < 1000);
	assert(0 < Y_a && Y_a < 1000);
	assert(0 < X_b && X_b < 1000);
	assert(0 < Y_b && Y_b < 1000);

	if(Y_a == Y_b){
		cout << Y_a << endl;
		return 0;
	}
	double a = (Y_a-Y_b)/(double)(X_a+X_b);
	double b = Y_a - a*X_a;
	printf("%.12f\n", b);
	return 0;
}