#include <iostream>
#include <vector>
#include <cmath>
#include <iomanip>

using namespace std;
//変数デバッグ
#define DEB(variable) cout << #variable << "=" << variable << endl

//for簡易表記(引数ミス防止)
#define INFOR(inc, a, b) for (int inc = (a); (inc) < (b); (inc)++) //incをaにし、b-a回(iがb未満の間)処理を行う
#define DEFOR(inc, a, b) for ((inc) = (a); (inc) > (b); (inc)--)   //incをaにし、a-b回(iがbより大きい間)処理を行う

int main()
{
	int ax, ay, bx, by;
	cin >> ax >> ay >> bx >> by;

	cout <<setprecision(20)<< (fabs(ax - bx) + fabs(ay - by)) / 2.0 << endl;



	return 0;
}