import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.stream.Stream;

import static java.lang.System.in;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        String[] inputs = reader.readLine().split(" ");
        int px = Integer.parseInt(inputs[0]);
        int py = Integer.parseInt(inputs[1]);
        inputs = reader.readLine().split(" ");
        int qx = Integer.parseInt(inputs[0]);
        int qy = Integer.parseInt(inputs[1]);

        System.out.println((Math.abs(px - qx) + Math.abs(py - qy)) / 2.0);

    }
}