#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"

ll xa, ya, xb, yb;

long double f(long double v) {
    long double dx1 = abs(xa - 0);
    long double dy1 = abs(ya - v);
    long double dx2 = abs(xb - 0);
    long double dy2 = abs(yb - v);
    return sqrt(dx1 * dx1 + dy1 * dy1) + sqrt(dx2 * dx2 + dy2 * dy2);
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    
    cin >> xa >> ya >> xb >> yb;
    long double low = 0, high = 2000;
    rep(i, 100) {
        long double lmid = (low * 2 + high) / 3;
        long double hmid = (low + high * 2) / 3;
        if (f(lmid) > f(hmid)) {
            low = lmid;
        }
        else {
            high = hmid;
        }
    }
    printf("%.10Lf\n", low);
    return 0;
}