#include <stdio.h>

int main(void){
	double a,px=0, py=0, qx=0, qy=0;
	a = ((px-qx+px-py)/2);
	scanf("%f%f%f%f", &px, &py, &qx, &qy);

	if(a>0)
		printf("%f\n", a);
	else
		printf("%f\n", a *= -1);

	return 0;
}