#include<iostream>

int main() {
	int ax, ay, bx, by;

	std::cin >> ax >> ay >> bx >> by;

	if (ax < 0) ax = -ax;
	if (ay < 0) ay = -ay;
	if (bx < 0) bx = -bx;
	if (by < 0) by = -by;


	std::cout << (ax + bx) / 2.0 + (ay + by) / 2.0 << std::endl;
	return 0;
}