import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		double px, py, qx, qy, ans;
		px = sc.nextDouble();
		py = sc.nextDouble();
		qx = sc.nextDouble();
		qy = sc.nextDouble();
		ans = Math.abs((px - qx) / 2) + Math.abs((py - qy) / 2);
		System.out.println(ans);
	}
}