#include <bits/stdc++.h>

//const static double	de_PI	= 3.14159265358979323846;
//const static int	de_MOD	= 1000000007;
//const static int	de_MAX	= 999999999;
//const static int	de_MIN = -999999999;

inline std::complex<double> Linear_Function(std::complex<double> &A, std::complex<double> &B) {

	std::complex<double> result;
	result.real((B.imag() - A.imag()) / (B.real() - A.real()));
	result.imag(A.imag() - result.real()*A.real());
	return(result);
}

int main(void) {

	//std::ifstream in("123.txt");	std::cin.rdbuf(in.rdbuf());

	std::complex<double> A, B, ans;
	double temp = 0;
	std::cin >> temp; A.real(temp);
	std::cin >> temp; A.imag(temp);
	std::cin >> temp; B.real(temp);
	std::cin >> temp; B.imag(temp);

	if (A.imag() == B.imag()) {
		std::cout << A.imag() << std::endl;
		return 0;
	}
	if (A.real() == B.real()) {
		std::cout << (A.imag()+B.imag()) / 2.0 << std::endl;
		return 0;
	}

	A.real(A.real()*-1);
	ans = Linear_Function(A, B);
	std::cout << std::fixed << std::setprecision(10) << ans.imag() << std::endl;

}