#include <bits/stdc++.h>

using namespace std;

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	long long int x1,x2,y1,y2;

	cin >> x1 >> y1;
	cin >> x2 >> y2;

	long long int dx = abs(x1-x2);
	long long int dy = abs(y1-y2);

	long double res = (dx/2.0) + (dy/2.0);

	cout << fixed;
	cout.precision(6);
	cout << res << '\n';

	return 0;
}