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

int min(int arg1, int arg2) {
	return (arg1 < arg2) ? arg1 : arg2;
}

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

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

	r = min(abs(px - qx), abs(py - qy))
		+ abs(abs(px - qx) - abs(py - qy)) / 2.0;
	
	printf("%.1lf", r);

	return 0;
}