#include <stdio.h>
#include <stdlib.h>

int main()
{
	int Px, Py, Qx, Qy, p, q;
	float ans;

	if (scanf("%d%d%d%d", &Px, &Py, &Qx, &Qy) != 4) {
		return 1;
	}

	p = abs(Px - Qx);
	q = abs(Py - Qy);

	ans = (float)(p + q) / 2;

	printf("%.1f", ans);

	return 0;
}