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

int main() {
	pair<int64_t, int64_t> a, b;
	cin >> a.first >> a.second >> b.first >> b.second;

	int d = max(a.first, b.first) - min(a.first, b.first);
	d += max(a.second, b.second) - min(a.second, b.second);
	double r = d / 2.0;
	cout << fixed << setprecision(20) << r << endl;
}