#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;

	printf("%.20f\n", abs(Px - Qx) / 2.0 + abs(Py - Qy) / 2.0);
}