import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int Px = scan.nextInt();
		int Py = scan.nextInt();
		int Qx = scan.nextInt();
		int Qy = scan.nextInt();
		scan.close();
		int k1 = Px - Qx;
		int k2 = Py - Qy;
		if(k1 < 0) {
			k1 = - k1;
		}
		if(k2 < 0) {
			k2 = - k2;
		}

		double ans = (k1 + k2) / 2.0;
		System.out.println(ans);
	}
}