#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main()
{
	int px, py, qx, qy;

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

	cout << fixed << setprecision(1) << (fabs(px -qx) + fabs(py-qy)) / 2 << endl;

    return 0;
}