import java.util.Scanner;

class no410{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		double a,b,c,d,t;
		a=stdIn.nextDouble();
		b=stdIn.nextDouble();
		c=stdIn.nextDouble();
		d=stdIn.nextDouble();
		if(c-a<0) a-=c;
		else a=c-a;
		if(d-b<0) b-=d;
		else b=d-b;
		System.out.print((a+b)/2.);
	}
}