#include<stdio.h>
#include<stdlib.h>

int main(void){

	int Px,Py,Qx,Qy;
	double a;

	scanf("%d %d %d %d",&Px,&Py,&Qx,&Qy);
	
	a = (double)abs(Px-Qx)/2+(double)abs(Py-Qy)/2;
	printf("%f",a);

	return 0;

}