//No.410 出会い

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

int main()
{
int i;
int PX;
int PY;
int QX;
int QY;
int X;
int Y;

	cin >> PX;
	cin >> PY;
	cin >> QX;
	cin >> QY;

	X = PX - QX;
	Y = PY - QY;
	if( X < 0 ) X *= - 1;
	if( Y < 0 ) Y *= - 1;

	i = X + Y;
	if( i % 2 == 0 )
		cout << i / 2 << endl;
	else
		cout << i / 2 << ".5" << endl;

	return 0;
}