#include <iostream>

int main() {
	int ax, ay, bx, by;
	std::cin >> ax >> ay >> bx >> by;

	ax = bx - ax;
	ay = by - ay;

	if (ax < 0) ax *= -1;
	if (ay < 0) ay *= -1;

	std::cout << (ax + ay) / 2.0 << std::endl;

	return 0;
}