using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	const decimal eps=(decimal)1e-11;
	static int[] a,b;
	static void Main(){
		Sc sc=new Sc();
		a=sc.Ia;
		b=sc.Ia;
		if(a[1]>b[1]){
			a[0]^=b[0];b[0]^=a[0];a[0]^=b[0];
			a[1]^=b[1];b[1]^=a[1];a[1]^=b[1];
		}
		Console.WriteLine(Gss(a[1]-1,b[1]+1));
	}
	static readonly decimal phi=(1+Sqrt(5))*(decimal)0.5;
	static decimal Gss(decimal a,decimal b){
		decimal lb=a,ub=b,c1=0,c2=0,d1=0,d2=0;
		c1=(lb*phi+ub)/(1+phi);
		c2=(lb+phi*ub)/(1+phi);
		bool bo=true;
		d1=Fu(c1);
		while(ub-lb>eps){
			if(bo){d2=Fu(c2);}
			else{d1=Fu(c1);}
			if(d1>d2){lb=c1;c1=c2;c2=(lb+phi*ub)/(1+phi);d1=d2;bo=true;}
			else{ub=c2;c2=c1;c1=(lb*phi+ub)/(1+phi);d2=d1;bo=false;}
		}
		return c1;
	}
	static decimal Fu(decimal c){
		return Sqrt(a[0]*a[0]+(a[1]-c)*(a[1]-c))+Sqrt(b[0]*b[0]+(b[1]-c)*(b[1]-c));
	}
	static decimal Sqrt(decimal x,decimal? guess=null){
		var ourGuess=guess.GetValueOrDefault(x/2m);
		var result=x/ourGuess;
		var average=(ourGuess+result)/2m;
		if(average==ourGuess){return average;}
		else{return Sqrt(x,average);}
	}
}

public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public double[] Da2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),double.Parse);}}
	public double[] Da3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),double.Parse);}
	public double[] Da3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),double.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}