#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <iostream>
#include <iomanip>
#include <vector>
#include <string>
#include <sstream>
#include <algorithm>
#include <utility>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <functional>

using namespace std;

#define double long double

#define fst first
#define scd second
#define PB push_back
#define MP make_pair
#define rep(i,x) for(int i=0;i<(x);++i)
#define rep1(i,x) for(int i=1;i<=(x);++i)
#define rrep(i,x) for(int i=(x)-1;i>=0;--i)
#define rrep1(i,x) for(int i=(x);i>=1;--i)
#define int long long
#define FOR(i,a,x) for(int i=(a);i<(x);++i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define omajinai ios::sync_with_stdio(false);cin.tie(0)

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pii;
typedef vector<pii> vpii;

template<typename T>T& max(T&a,T&b){if(a>=b)return a;return b;}
template<typename T>T& min(T&a,T&b){if(a<b)return a;return b;}
template<typename T>bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<typename T>bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<typename T>T get(){T a;cin>>a;return a;}
template<typename T>T rev(T a){reverse(all(a));return a;}
template<typename T>istream&operator>>(istream&is,vector<T>&vec){rep(i,vec.size())is>>vec[i];return is;}
template<typename T>vector<T>&sort(vector<T>&a){sort(all(a));return a;}

const int inf = 1e9;
const ll linf = 3e18;
const double eps = 1e-9;

struct P {
    double x, y;
    P(){}
    P(double x, double y):x(x),y(y){}
};

double f(P p0, P p1, P p2)
{
    double AP = sqrt(pow(p0.x - p1.x, 2) + pow(p0.y - p1.y, 2));
    double PB = sqrt(pow(p0.x - p2.x, 2) + pow(p0.y - p2.y, 2));
    return AP + PB;
}

signed main()
{
    P a, b;

    cin >> a.x >> a.y;
    cin >> b.x >> b.y;

    if (a.y > b.y) {
        P tmp = b;
        b = a;
        a = tmp;
    }

    double lb = a.y, ub = b.y;
    
    rep(_, 100) {
        if (f(P{0, (lb * 2 + ub) / 3}, a, b) < f(P{0, (lb + ub * 2) / 3}, a, b)) {
            ub = (lb + ub * 2) / 3;
        }  else {
            lb = (lb * 2 + ub) / 3;
        }
    }

    cout << fixed << setprecision(6) << (lb + ub) / 2 << endl;
}