import java.util.Scanner;
public class Main{
	private static Scanner sc=new Scanner(System.in);
	public static void main(String args[])throws Exception{
		int x1=sc.nextInt(),y1=sc.nextInt(),x2=sc.nextInt(),y2=sc.nextInt();
		int distance=Math.abs(x2-x1)+Math.abs(y2-y1);
		System.out.println(distance*1.0/2);
	}
}