// template {{{

#include <bits/stdc++.h>
using namespace std;

// #define int long long

#define GET_MACRO(a, b, c, d, NAME, ...) NAME
#define REP2(i, n) REP3(i, 0, n)
#define REP3(i, a, b) REP4(i, a, b, 1)
#define REP4(i, a, b, s) for (int i = (a); i < (int)(b); i += s)
#define RREP2(i, n) RREP3(i, 0, n)
#define RREP3(i, a, b) for (int i = (b) - 1; i >= (int)(a); i--)

#define rep(...) GET_MACRO(__VA_ARGS__, REP4, REP3, REP2)(__VA_ARGS__)
#define rrep(...) GET_MACRO(__VA_ARGS__,, RREP3, RREP2)(__VA_ARGS__)
#define eb emplace_back
#define ef emplace_front
#define pb pop_back
#define pf pop_front
#define all(c) std::begin(c), std::end(c)
#define fi first
#define se second
#define popcnt __builtin_popcountll

using uint = unsigned;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using vi = std::vector<int>;
using vvi = std::vector<vi>;

template<typename T>
using maxpq = std::priority_queue<T, std::vector<T>, std::less<T>>;

template<typename T>
using minpq = std::priority_queue<T, std::vector<T>, std::greater<T>>;

const int INF = 1e9 + 10;
const ll LLINF = 1e18 + 10;

const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};
const int dx8[] = {-1, -1, 0, 1, 1, 1, 0, -1};
const int dy8[] = {0, -1, -1, -1, 0, 1, 1, 1};

template<typename T>
inline T sq(T x){ return x * x; }

template<typename T, typename U>
inline bool chmax(T &x, U y){ if (x >= y) return false; x = y; return true; }

template<typename T, typename U>
inline bool chmin(T &x, U y){ if (x <= y) return false; x = y; return true; }

template<typename T>
inline T& sort(T &c){ sort(all(c)); return c; }

template<typename T>
inline T& reverse(T &c){ reverse(all(c)); return c; }

template<typename T>
inline T& rsort(T &c){ sort(all(c), greater<T>()); return c; }

template<typename T>
inline T& unique(T &c){ sort(all(c)); c.erase(unique(all(c)), end(c)); return c; }

// }}}

const double phi = (1.0 + sqrt(5.0)) / 2.0;

double xa, ya, xb, yb;

double dist(double y)
{
	return hypot(xa, y - ya) + hypot(xb, y - yb);
}

pair<double, double> sep(double lb, double ub)
{
	return make_pair(
		(lb * phi + ub) / (1.0 + phi),
		(lb + ub * phi) / (1.0 + phi)
	);
}

int main()
{
	cin >> xa >> ya >> xb >> yb;

	double lb = -10.0, ub = 1010.0;
	double x1, x2, y1, y2;
	tie(x1, x2) = sep(lb, ub);
	y1 = dist(x1), y2 = dist(x2);
	rep(i, 100){
		if (y1 > y2){
			lb = x1;
			y1 = y2;
			y2 = dist(sep(lb, ub).second);
			tie(x1, x2) = sep(lb, ub);
		}
		else {
			ub = x2;
			y2 = y1;
			y1 = dist(sep(lb, ub).first);
			tie(x1, x2) = sep(lb, ub);
		}
	}

	printf("%.20f\n", lb);
}