#include<stdio.h>

int dist(int x,int y){
	if(x>y){return x-y;}
	else{return y-x;}
}

int main(void){
	int px,py,qx,qy,a,b;
	double r;
	scanf("%d%d",&px,&py);
	scanf("%d%d",&qx,&qy);
	a = dist(px,qx);
	b = dist(py,qy);
	r = a+b;
	printf("%lf\n",r/2);
	return 0;
}