#include <cstdio>
#include <iostream>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <string>
#include <cstring>
#include <sstream>
#include <algorithm>
#include <functional>
#include <queue>
#include <stack>
#include <cmath>
#include <iomanip>
#include <list>
#include <tuple>
#include <bitset>
#include <ciso646>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> T;
typedef vector<ll> vec;

inline bool cheak(ll x, ll y, ll xMax, ll yMax){ return x >= 0 && y >= 0 && xMax > x && yMax > y; }
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
template<class _RanIt> inline string toString(_RanIt _First, _RanIt _Last, string sep) { if (_First == _Last)return ""; ostringstream sout; sout << *_First; for (_First = next(_First); _First != _Last; _First = next(_First)) { sout << sep << *_First; }return sout.str(); }
template<class _RanIt> inline string toString(_RanIt _First, _RanIt _Last, string sep, string sep2) { if (_First == _Last)return ""; ostringstream sout; sout << toString(all(*_First), sep); for (_First = next(_First); _First != _Last; _First = next(_First)) { sout << sep2 << toString(all(*_First), sep); }return sout.str(); }
template<class T> inline T sqr(T x) { return x*x; }
template<class T> inline T mypow(T x, ll n){ T res = 1; while (n > 0){ if (n & 1)res = res * x;	x = x * x;	n >>= 1; }return res; }
inline int gcd(ll a, ll b){ return b ? gcd(b, a%b) : a; }
inline int lcm(ll a, ll b){ return a / gcd(a, b) * b; }

#define For(i,a,b)	for(ll (i) = (a);i < (b);(i)++)
#define rep(i,n)	For(i,0,n)
#define rFor(i,a,b)	for(ll (i) = (a-1);i >= (b);(i)--)
#define rrep(i,n)	rFor(i,n,0)
#define clr(a)		memset((a), 0 ,sizeof(a))
#define mclr(a)		memset((a), -1 ,sizeof(a))
#define all(a)		(a).begin(),(a).end()
#define sz(a)		(sizeof(a))
#define tostr(a)	toString(a)

const ll dx[8] = { 1, 0, -1, 0, 1, 1, -1, -1 }, dy[8] = { 0, -1, 0, 1, -1, 1, -1, 1 };

const ll mod = 1e9 + 7;
const ll INF = 1e13 + 9;

#define int ll

signed main(){
	ld xa, ya;
	ld xb, yb;
	cin >> xa >> ya >> xb >> yb;
	
	ld l = -INF, r = INF;
	while (1){
		ld y1 = l + (r - l) / 3.0;
		ld y2 = r - (r - l) / 3.0;

		ld l1 = sqrt(sqr(y1 - ya) + sqr(xa)) + sqrt(sqr(y1 - yb) + sqr(xb));
		ld l2 = sqrt(sqr(y2 - ya) + sqr(xa)) + sqrt(sqr(y2 - yb) + sqr(xb));
		
		if (l1 > l2){
			l = y1;
		}
		else{
			r = y2;
		}
		if (r - l < 1e-10)break;
	}
	cout << fixed << setprecision(30) << l << endl;

	return 0;
}