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

double xa, ya, xb, yb, l, r;

inline double d(double x1, double y1, double x2, double y2){
  return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}

int main(){
  cin >> xa >> ya >> xb >> yb;
  l=-1000, r=1000;
  while(r-l>1e-9){
    double ml=l+(r-l)/3, mr=r-(r-l)/3;
    if(d(xa,ya,0,ml)+d(xb,yb,0,ml)<d(xa,ya,0,mr)+d(xb,yb,0,mr)){
      r=mr;
    }else{
      l=ml;
    }
  }

  cout<<fixed<<setprecision(16);
  cout << l << endl;
}