import java.util.Scanner;
public class Main {
		public static void main(String[] args) {
			Scanner sc = new Scanner(System.in);
			int Px = sc.nextInt();
			int Py = sc.nextInt();
			int Qx = sc.nextInt();
			int Qy = sc.nextInt();
			
			double PQx = (Px + Qx) / 2.0;
			double PQy = (Py + Qy) / 2.0;
			
			double t = Math.abs(Px - PQx) + Math.abs(Py - PQy);
			
			System.out.printf("%.20f\n",t);
			
		}
}