import java.util.Scanner;
import java.util.ArrayList;
import java.util.regex.*;
import java.util.Collections;
import java.util.*;
import java.math.*;
import java.util.Stack;



public class woo_01 {
    

    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);


        // ArrayList<String> box = new ArrayList<String>();
        // int n = Integer.parseInt(sc.nextLine());
        // Deque<String> que = new ArrayDeque<String>();
        int[] pos_a = new int[2];
        int[] pos_b = new int[2];
        String  line = sc.nextLine();
        inSpaceDelimiterInt(line, pos_a);
        line = sc.nextLine();
        inSpaceDelimiterInt(line, pos_b);

        int delta_x = Math.abs(pos_a[0] - pos_b[0]);
        int delta_y = Math.abs(pos_a[1] - pos_b[1]);

// System.out.println("delata_x="+delta_x+" delata_y="+delta_y);
        double meet_x = (double)delta_x / 2;
        double meet_y = (double)delta_y / 2;
// System.out.println("meet_x="+meet_x+" meet_y="+meet_y);

        // if ( meet_x > meet_y )
            // System.out.println(meet_x);
        // else
            System.out.println(meet_x+meet_y);
    }
   
    private static void inSpaceDelimiterInt(String readLine, int[] box) {
		// String readLine = br.readLine();
    	readLine = readLine.trim();	
		String[] readLineArray = readLine.split(" ");
        try{
            for( int i=0; i<readLineArray.length; i++) {
                box[i] = Integer.parseInt(readLineArray[i]);
            }
		} catch (Exception e) {
            System.out.println("Illgal number");
        }		
		return ;
	}

	private static void inSpaceDelimiterString(String readLine, ArrayList<String> box) {
		// String readLine = br.readLine();
    	readLine = readLine.trim();	
		String[] readLineArray = readLine.split(" ");
        try{
            for( int i=0; i<readLineArray.length; i++) {
                box.add(readLineArray[i]);
            }
		} catch (Exception e) {
            System.out.println("Illgal number");
        }		
		return ;
	}

	private static void inSpaceDelimiterIntArrayList(String readLine, ArrayList<Integer> box) {
		// String readLine = br.readLine();
    	readLine = readLine.trim();	
		String[] readLineArray = readLine.split(" ");
        try{
            for( int i=0; i<readLineArray.length; i++) {
                box.add(Integer.parseInt(readLineArray[i]));
            }
		} catch (Exception e) {
            System.out.println("Illgal number");
        }		
		return ;
	}
}