import java.util.*;

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 ans = (double)(px + qx) / (double)2 + (double)(py + qy) / (double)2;
    System.out.println(ans);
  }
}