#include<stdio.h>

int zettai(int num){
	return num>=0 ? num:-num;
}

int main(){
	int px,py,qx,qy;
	double ans;

	scanf("%d %d %d %d",&px,&py,&qx,&qy);

	ans = (zettai(px-qx) + zettai(py-qy)) / 2.0;

	printf("%.1f\n",ans);

	return 0;
}