#include <bits/stdc++.h>
using namespace std;
struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define vv(type, c, m, n, i) vector<vector<type>> c(m, vector<type>(n, i))
#define rep(i,a,n) for(int i=(a), i##_len=(n); i<i##_len; ++i)
#define rrep(i,a,n) for(int i=(a), i##_len=(n); i>i##_len; --i)
#define each(x,y) for(auto &(x):(y))
#define var(type, ...)type __VA_ARGS__;Scan(__VA_ARGS__);
template<typename T> void Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void Scan(First& first, Rest&...rest) { cin >> first; Scan(rest...); }
#define vec(type, c, n) vector<type> c(n);for(auto& i:c) cin>>i;
#define len(x) ((int)(x).size())
#define mp make_pair
#define eb emplace_back
#define em emplace
#define pb pop_back
#define fi first
#define se second
#define get(a, i) get<i>(a)
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long long> vll;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef pair<int, int> pii;
#define mini min_element
#define maxi max_element
#define sum(a, b) accumulate(a, b)
#define chmax(a,b) if(a<b)a=b
#define chmin(a,b) if(a>b)a=b
template <class T> inline void print(T t){cout << t << "\n";}
template <class H, class... T> inline void print(H h, T... t){cout << h << " ";print(t...);}
double solve() {
	var(double, xa, ya, xb, yb);
	double low = min(ya, yb), high = max(ya, yb);
	double mid, t1, t2, t;
	map<double, double> m;
	m[ya] = xa + hypot(xb, abs(ya - yb));
	m[yb] = xb + hypot(xa, abs(ya - yb));
	rep(i, 0, 100) {
		mid = (low + high) / 2;
		t1 = hypot(xa, abs(mid - ya));
		t2 = hypot(xb, abs(mid - yb));
		t = t1 + t2;
		m[mid] = t;
		if (abs(m[low] - m[mid]) > abs(m[high] - m[mid])) {
			low = mid;
		} else {
			high = mid;
		}
	}
	return mid;
}
int main() {
	cout << setprecision(12) << fixed;
	print(solve());
	return 0;
}