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


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


int main(){

  long double ax,ay,bx,by;
  cin >> ax >> ay >> bx >> by;

  long double l,r;
  l = min(ay,by);
  r = max(ay,by);

  for(int i=0;i<100;i++){
    long double c1 = l + (r-l)/3.0;
    long double c2 = l + (r-l)*2.0/3.0;
    long double f1 = d(ax,ay,0,c1) + d(bx,by,0,c1);
    long double f2 = d(ax,ay,0,c2) + d(bx,by,0,c2);
    if(f1 > f2) {
      l = c1;
    }else{
      r = c2;
    }
  }
  cout<<fixed<<setprecision(10) << r << endl;
}