#include<stdio.h>

#include <stdlib.h>


int main(void){
	int px,py,qx,qy;
	double total;
	scanf("%d %d %d %d",&px,&py,&qx,&qy);

	printf("%.1f\n",(abs(px-qx)+abs(py-qy))/2.0);
			
	return 0;

}