#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) for(int i = (n) - 1; (i) >= 0; --i)
#define SZ(n) (int)(n).size()
#define ALL(n) (n).begin(), (n).end()
#define MOD LL(1e9 + 7)
#define INF 1000000
using namespace std;
typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PI;

int main() {
    int x[2];
    int y[2];
    cin >> x[0] >> y[0];
    cin >> x[1] >> y[1];
    double ans = 0;
    double r = 0;
    double l = 1000;
    REP(i, 100) {
        if(hypot(x[0], y[0] - l) + hypot(x[1], y[1] - l) > hypot(x[0], y[0] - r) + hypot(x[1], y[1] - r)) 
            l = (r + l) / 2;
        else
            r = (r + l) / 2;
    }
    ans = (hypot(x[0], y[0] - l) + hypot(x[1], y[1] - l) > hypot(x[0], y[0] - r) + hypot(x[1], y[1] - r) ? r : l);
    cout << ans << endl;
    return 0;
}