import java.util.Scanner;

public class a{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
		double ans;
        double a = sc.nextDouble();
        double b = sc.nextDouble();
		double c = sc.nextDouble();
		double d = sc.nextDouble();
		ans=Math.abs(c-a)/2+Math.abs(d-b)/2;
        System.out.println(ans);
    }
}