import java.util.*;

public class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		int x1 = sc.nextInt();
		int y1 = sc.nextInt();
		int x2 = sc.nextInt();
		int y2 = sc.nextInt();
		double ans = (Math.abs(x2-x1)+Math.abs(y2-y1))/(double)2;
		System.out.println(ans);
	}
}