import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class No00000410_Main2 {
	static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	public static void main(String[] args) throws IOException {
		String[] p = br.readLine().split(" ");
		String[] q = br.readLine().split(" ");
		System.out.println((Math.abs(Integer.parseInt(p[0]) - Integer.parseInt(q[0])) + Math.abs(Integer.parseInt(p[1]) - Integer.parseInt(q[1]))) / 2.0);
	}
}