#include <stdio.h>

#define abs(x) (x > 0 ? x : -(x))

int px, py, qx, qy;

int main() {
	scanf("%d%d%d%d", &px, &py, &qx, &qy);
	float ans = ( abs(px - qx) + abs(py - qy) ) / 2.0;
	
	printf("%.1f\n", ans);
	
	return 0;
}