import java.util.Scanner;

public class b {
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int px = sc.nextInt();
		int py = sc.nextInt();
		int qx = sc.nextInt();
		int qy = sc.nextInt();
		double foo = px - qx;
		double bar = py - qy;
		System.out.println((Math.abs(foo) + Math.abs(bar))/2);
	}
}