#include <iostream>
#include <cstdio>
#include <algorithm>
#include <utility>

using namespace std;

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

	cin >> px >> py >> qx >> qy;
	double ans = abs(px - qx) + abs(py - qy);
	ans /= 2.0;
	printf("%.1f\n", ans);
	return 0;
}