#include <iostream>
#include <iomanip>
using namespace std;
int main() {
	int Px, Py, Qx, Qy;
	double x_diff, y_diff;
	cin >> Px >> Py >> Qx >> Qy;
	x_diff = (Px - Qx > 0) ? Px - Qx : Qx - Px;
	y_diff = (Py - Qy > 0) ? Py - Qy : Qy - Py;
	cout << fixed << (x_diff + y_diff) / 2.0 << endl;
	return 0;
}