#define _USE_MATH_DEFINES
 
#include <iostream>
#include <iomanip>
#include <sstream>
 
#include <algorithm>
#include <cmath>
 
#include <string>
#include <queue>
#include <vector>
#include <complex>
#include <set>
#include <map>
#include <stack>
#include <list>

#include <valarray>

#include<cassert>//assert();
//#include <random>//xAOJ
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;
 
#define PII pair<int,int>
/////////
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
/////////
using namespace::std;
/////////
/////////
LD kyori(LD Ax,LD Ay,LD Bx,LD By){
	return sqrt( (Bx-Ax)*(Bx-Ax) + (By-Ay)*(By-Ay) );
}
void solve(){
	LD Ax,Ay;
	LD Bx,By;
	cin >> Ax >> Ay;
	cin >> Bx >> By;
	if( Ay == By ){
		cout << Ay << endl;
		return;
	}
	//3bun
	LD left,right;
	LD midL,midR;
	if( Ay < By ){
		left = Ay;
		right = By;
	}else{
		left = By;
		right = Ay;
	}
	LD Lval,Rval;
	int count = 2000000;
	while(count--){
		midL =(2*left + right)/3;
		midR =(left + 2*right)/3;
		Lval = kyori(0,midL,Ax,Ay) + kyori(0,midL,Bx,By);
		Rval = kyori(0,midR,Ax,Ay) + kyori(0,midR,Bx,By);
		if( Rval < Lval ){
			left = midL;
		}else{
			right = midR;
		}
	}
	cout << midL << endl;
}
int main(void){
    std::cin.tie(0); 
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed;//
    cout << setprecision(16);//
	
	//cpp
	solve();
	return 0;
}