#include <stdio.h>

int abs(int n){
	if(n < 0){
		n *= -1;
	}
	return n;
}

int main(){
	int px, py, qx, qy;
	scanf("%d %d %d %d", &px, &py, &qx, &qy);
	
	printf("%.20lf",(double)(abs(px - qx) + abs(py - qy)) / 2);	
	
}