#include <iostream>

int main(){
	std::cin.tie(0);
	std::ios::sync_with_stdio(false);

	double ax, ay, bx, by;
	std::cin >> ax >> ay >> bx >> by;
	bx = -bx;

	if(ax != bx)
		std::cout << -ax * ((ay - by) / (ax - bx)) + ay << std::endl;
	else
		std::cout << (ay + by) / 2 << std::endl;

	return 0;
}