#include <bits/stdc++.h>
using namespace std;
typedef long double D;

D TernarySearchLowestF(D L, D R, function<D(D)> F, D eps = 1e-9){
    D l = L, r = R;
    while(l < r && (r - l) > eps){
        D l2 = l + (r - l) / 3, r2 = r - (r - l) / 3;
        if(F(r2) < F(l2)) l = l2; else r = r2;
    }
    return (l+r)/2;
}

int main(){
    int xa,ya,xb,yb;
    cin >> xa >> ya >> xb >> yb;
    if(ya > yb){
        swap(xa,xb);swap(ya,yb);
    }
    D xa2 = xa*xa, xb2 = xb*xb;
    auto f = [&](D yp)->D{ return sqrt(xa2 + (ya-yp)*(ya-yp)) + sqrt(xb2 + (yb-yp)*(yb-yp)); };
    D res = TernarySearchLowestF(0.+ya, 0.+yb, f);

    cout << setprecision(12) << res << endl;
}