#include<cstdio>
#include<algorithm>
#include<vector>
#include<cmath>
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;}

int main(){
    double x1,x2,y1,y2;
    scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);

    double lb=0,ub=1000;
    for(int i=0;i<100;i++){
        double mid=(lb+ub)/2;

        double x=(mid-y1)/sqrt(x1*x1+(mid-y1)*(mid-y1))
                +(mid-y2)/sqrt(x2*x2+(mid-y2)*(mid-y2));

        if(x<0)lb=mid;
        else ub=mid;
    }

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

    return 0;
}