#include <bits/stdc++.h>
using namespace std;
int a, b, c, d;
int main() {
	cin >> a >> b >> c >> d;
	int r = abs(a - c) + abs(b - d);
	if(r % 2 == 1) cout << r / 2 << ".5" << endl;
	else cout << r / 2 << endl;
	return 0;
}