#include<iostream>
#include<math.h>
using namespace std;
int main()
{
	double px, py, qx, qy, s;

	cin >> px >> py >> qx >> qy;

	s = (fabs(px - qx) + fabs(py - qy))/2;

	//cout << s << endl;
	printf("%lf", s);

	return 0;
}