#include <bits/stdc++.h>
using namespace std;
int main() {
       	int px, py, qx, qy;
       	double x, y, t;
       	cin >> px >> py >> qx >> qy;
       	x = abs(px - qx);
       	x = x / 2.0;
       	y = abs(py - qy);
       	y = y / 2.0;
       	t = x + y;

       	printf("%.5f\n", t);

       	return 0;
}