#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
istream &operator>>(istream &is, modint &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint &a) { return os << a.val(); }
istream &operator>>(istream &is, modint998244353 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint998244353 &a) { return os << a.val(); }
istream &operator>>(istream &is, modint1000000007 &a) { long long v; is >> v; a = v; return is; }
ostream &operator<<(ostream &os, const modint1000000007 &a) { return os << a.val(); } 

typedef long long ll;
typedef vector<vector<int>> Graph;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define rep(i,n) for (int i = 0;i < (int)(n); i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define my_sort(x) sort(x.begin(), x.end())
#define my_max(x) *max_element(all(x))
#define my_min(x) *min_element(all(x))
#define cout_vector(x) rep(i,x.size()) cout << x[i] << ' ';
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int INF = (1<<30) - 1;
const ll LINF = (1LL<<62) - 1;
const int MOD = 998244353;
const int MOD2 = 1e9+7;
const double PI = acos(-1);
vector<int> di = {1,0,-1,0};
vector<int> dj = {0,1,0,-1};

long double xa,ya,xb,yb;

long double f(long double p){
    return pow(xa*xa+(ya-p)*(ya-p), 0.5) + pow(xb*xb+(yb-p)*(yb-p), 0.5);
}

int main(){
    cin.tie(0);
    ios_base::sync_with_stdio(false);
    cin >> xa >> ya;
    cin >> xb >> yb;
    long double l = 0, r = 1000;
    long double eps = 1e-9;
    while(abs(l-r) > eps){
        double t1 = (l * 2 + r) / 3;
        double t2 = (l + r * 2) / 3;
        if (f(t1) < f(t2)) r = t2;
        else l = t1;
    }
    cout << fixed << setprecision(10) << l << endl;
}