package No400番台;
import java.util.Scanner;
public class Q410 {
	public static void main(String[] args) {
		new Q410().solver();
	}

	void solver() {
		Scanner sc = new Scanner(System.in);
		double px, py, qx, qy;
		px = sc.nextDouble();
		py = sc.nextDouble();
		qx = sc.nextDouble();
		qy = sc.nextDouble();
		long t = 0;
		System.out.println(Math.abs((px - qx) / 2.0) + Math.abs((py - qy) / 2.0));
//		t = Math.abs((px - qx) / 2) + Math.abs((py - qy) / 2);
//		if ((px - qx) % 2 == 1 && (py - qy) % 2 == 1) {
//			System.out.println((t + 1));
//		} else if ((px - qx) % 2 == 0 && (py - qy) % 2 == 0) {
//			System.out.println(t);
//		} else {
//			System.out.println((t + 1));
//		}
	}
}