#include <cmath>
#include <iostream>

using namespace std;

int main(int argc, const char* argv[])
{
	int Px, Py, Qx, Qy;
	cin >> Px >> Py >> Qx >> Qy;
	cout << (double)(abs(Qx - Px) + abs(Qy - Py)) / 2 << endl;
	return 0;
}