#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
//#define int long long
using namespace std;

int Px, Py, Qx, Qy;

signed main()
{
	cin >> Px >> Py;
	cin >> Qx >> Qy;

	double ans = 0.0;

	if (abs(Px - Qx) % 2 == 0 && abs(Py - Qy) % 2 == 0)
	{
		ans = abs(Px - Qx) / 2.0 + abs(Py - Qy) / 2.0;
	} else if (abs(Px - Qx) % 2 == 0 && abs(Py - Qy) % 2 == 1) {
		ans = abs(Px - Qx) / 2.0 + abs(Py - Qy) / 2.0;
	} else if (abs(Px - Qx) % 2 == 1 && abs(Py - Qy) % 2 == 0) {
		ans = abs(Px - Qx) / 2.0 + abs(Py - Qy) / 2.0;
	} else if (abs(Px - Qx) % 2 == 1 && abs(Py - Qy) % 2 == 1) {
		ans = abs(Px - Qx) / 2.0 + abs(Py - Qy) / 2.0;
	}
	//何となく書いているうちに気づいてた・・・

	cout << ans << endl;
}