#include <stdio.h>
#include <math.h>

int main(void)
{
	double x[2],y[2];
	int i;
	for(i=0;i<2;i++)scanf("%lf%lf",&x[i],&y[i]);
	printf("%f",(fabs(x[1]-x[0])+fabs(y[1]-y[0]))/2);
	return 0;
}