#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
#define REP(i, n) for(int i=0; i<n; i++)
#define REPi(i, a, b) for(int i=int(a); i<int(b); i++)
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
const ll MOD = 1e9+7;

int xa, ya, xb, yb;

long double dist(long double x1, long double y1, long double x2, long double y2){
    return (y1-y2)*(y1-y2) + (x1-x2)*(x1-x2);
}

long double solve(long double p){
    long double AP = dist(0, p, xa, ya);
    long double BP = dist(0, p, xb, yb);
    return sqrt(AP) + sqrt(BP);
    //return AP + BP;
}

int main(){
    cin >> xa >> ya >> xb >> yb;

    long double left = -1.0;
    long double right = 2000.0;
    REP(i,1000){
        long double a1 = (2*left + right) / 3;
        long double a2 = (left + 2*right) / 3;
        long double s1 = solve(a1);
        long double s2 = solve(a2);
        if(s1 < s2){
            right = a2;
        }
        else if(s2 <= s1){
            left = a1;
        }
    }

    printf("%.20Lf\n", left);
    return 0;
}