#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
int main(void) {
	double px, py, qx, qy;
	cin >> px >> py >> qx >> qy;
	double diff = abs(px - qx) + abs(py - qy);
	diff /= 2;
	cout << fixed << setprecision(10) << diff << endl;
	return 0;
}