import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int Px = Math.abs(Integer.parseInt(sc.next()));
        int Py = Math.abs(Integer.parseInt(sc.next()));
        int Qx = Math.abs(Integer.parseInt(sc.next()));
        int Qy = Math.abs(Integer.parseInt(sc.next()));

        System.out.println(((double)(Px+Qx) / 2) + ((double)(Py+Qy) / 2));
    }
}