#include <stdio.h>
#include <algorithm>
using namespace std;
using ll = long long;

//410
int main() {
	int px, py, qx, qy, distance;
	scanf("%d %d %d %d", &px, &py, &qx, &qy);
	distance = abs(px - qx) + abs(py - qy);
	printf("%d", distance / 2);
	if (distance & 1)
		printf(".5");
	printf("\n");
}