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

int main() {
	int Px, Py, Qx, Qy;
	double Dis;
	cin >> Px >> Py >> Qx >> Qy;
	Dis = abs(Px - Qx) + abs(Py - Qy);
	double ans = Dis / 2;
	printf("%.1f", ans);
}