import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Set;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int Px = sc.nextInt();
		final int Py = sc.nextInt();
		final int Qx = sc.nextInt();
		final int Qy = sc.nextInt();
		
		System.out.printf("%.8f\n", (Math.abs(Px - Qx) + Math.abs(Py - Qy)) / 2.0);
		
	}

}