// No.306 さいたま2008
// https://yukicoder.me/problems/no/306
//

#include <iostream>
#include <cmath>
#include <algorithm>
#include <iomanip>
using namespace std;

long double solve(long double ax, long double ay, long double bx, long double by);
long double calc_dist(long double ax, long double ay, long double bx, long double by, long double py);


int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    long double ax, ay, bx, by;
    cin >> ax >> ay >> bx >> by;
    long double ans = solve(ax, ay, bx, by);
    cout << fixed << setprecision(10) << ans << endl;
}

long double solve(long double ax, long double ay, long double bx, long double by) {
    long double uy = max(ay, by);
    long double ly = min(ay, by);

    for (auto i = 0; i < 100; ++i) {
        long double py1 = (ly * 2 + uy) / 3;
        long double mid1_dist = calc_dist(ax, ay, bx, by, py1);
        long double py2 = (ly + uy * 2) / 3;
        long double mid2_dist = calc_dist(ax, ay, bx, by, py2);

        if (mid2_dist > mid1_dist)
            uy = py2;
        else
            ly = py1;
    }

    return (uy + ly) / 2;
}

long double calc_dist(long double ax, long double ay, long double bx, long double by, long double py) {
    long double ap = sqrt((ax*ax + (ay-py)*(ay-py)));
    long double bp = sqrt((bx*bx + (by-py)*(by-py)));
    return ap + bp;
}