import java.util.*;

public class Exercise157 {
  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();

    int sum = Math.abs(pX - qX) + Math.abs(pY - qY);
    System.out.println((double)sum / 2);
  }
}