import java.util.*;
import java.util.stream.Stream;

public class No410 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
		int px = sc.nextInt(), py = sc.nextInt();
		int qx = sc.nextInt(), qy = sc.nextInt();
		System.out.println((Math.abs(px - qx) + Math.abs(py - qy)) / 2.0);
	}
}