#include<bits/stdc++.h>
using namespace std;

//#define int long long

typedef pair<int,int>pint;
typedef vector<int>vint;
#define pb push_back
#define mp make_pair
#define all(v) (v).begin(),(v).end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
template<class T,class U>void chmin(T &t,U f){if(t>f)t=f;}
template<class T,class U>void chmax(T &t,U f){if(t<f)t=f;}

typedef complex<double>Point;

Point get(){
    double x,y;
    scanf("%lf%lf",&x,&y);
    return Point(x,y);
}

int main(){
    Point A,B;
    A=get();B=get();

    double lb=0,ub=1000;

    for(int i=0;i<100;i++){
        double m1=(lb*2+ub)/3,m2=(lb+ub*2)/3;
        Point p1(0,m1),p2(0,m2);
        double d1=abs(A-p1)+abs(B-p1);
        double d2=abs(A-p2)+abs(B-p2);
        if(d1>d2)lb=m1;
        else ub=m2;
    }

    printf("%.20lf\n",ub);
    return 0;
}