import java.util.Scanner;

public class Main{

	static Scanner s=new Scanner(System.in);

	public static void main(String __[]){
		input();
		solve();
	}

	private static void input(){
	}
	private static void solve(){
		int px=s.nextInt(),py=s.nextInt(),qx=s.nextInt(),qy=s.nextInt();
		System.out.println((Math.abs(px-qx)+Math.abs(py-qy))/2.0);
	}
}