#include <bits/stdc++.h>
#define GET_MACRO(a, b, c, NAME, ...) NAME
#define rep(...) GET_MACRO(__VA_ARGS__, rep3, rep2)(__VA_ARGS__)
#define rep2(i, a) rep3 (i, 0, a)
#define rep3(i, a, b) for (int i = (a); i < (b); i++)
#define repr(...) GET_MACRO(__VA_ARGS__, repr3, repr2)(__VA_ARGS__)
#define repr2(i, a) repr3 (i, 0, a)
#define repr3(i, a, b) for (int i = (b) - 1; i >= (a); i--)
#define chmin(a, b) ((b) < a && (a = (b), true))
#define chmax(a, b) (a < (b) && (a = (b), true))
using namespace std;
typedef long long ll;

int main() {
	double l = -1e6, r = 1e6;
	double x1, y1, x2, y2;
	cin >> x1 >> y1 >> x2 >> y2;
	rep (i, 5000) {
		double m1 = (l * 2 + r) / 3;
		double m2 = (l + r * 2) / 3;
		double f1 = hypot(x1, y1 - m1) + hypot(x2, y2 - m1);
		double f2 = hypot(x1, y1 - m2) + hypot(x2, y2 - m2);
		if (f1 < f2) {
			r = m2;
		} else {
			l = m1;
		}
	}
	printf("%.20f\n", l);
	return 0;
}