#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

int main(){
    int xa, ya, xb, yb;
    cin >> xa >> ya >> xb >> yb;
    double l = min(ya, yb), r = max(ya, yb), ml, mr;
    rep(i, 100000){
        ml = (l * 2 + r) / 3;
        mr = (l + r * 2) / 3;
        double L, R;
        L = hypot(ml - ya, xa) + hypot(ml - yb, xb);
        R = hypot(mr - ya, xa) + hypot(mr - yb, xb);
        if(L >= R)l = ml;
        else r = mr;
    }
    cout << setprecision(20) << l << endl;
}