package me.yukicoder.lv1;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class No0410 {
	public static void main(String[] args) {
		try (BufferedReader br = new BufferedReader(new InputStreamReader(System.in))) {
			String[] str1 = br.readLine().split(" ");
			String[] str2 = br.readLine().split(" ");
			int[] p = { Integer.parseInt(str1[0]), Integer.parseInt(str1[1]) };
			int[] q = { Integer.parseInt(str2[0]), Integer.parseInt(str2[1]) };

			double dist = (Math.abs(p[0] - q[0]) + Math.abs(p[1] - q[1])) / 2.0;
			System.out.println(dist);
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
}