#include <bits/stdc++.h>
using namespace std;

int main()
{
	int Px, Py, Qx, Qy;
	cin >> Px >> Py >> Qx >> Qy;

	double ans = abs( Px - Qx ) + abs( Py - Qy );
	ans /= 2;

	cout << fixed << setprecision(5) << ans << endl;
}