There is a polyline going through points (0, 0) – (x, x) – (2x, 0) – (3x, x) – (4x, 0) – ... - (2kx, 0) – (2kx + x, x) – .... 

We know that the polyline passes through the point (a, b). Find minimum positive value x such that it is true or determine that there is no such x.

Input

Only one line containing two positive integers a and b (1 ≤ a, b ≤ 109).

Output

Output the only line containing the answer. Your answer will be considered correct if its relative or absolute error doesn't exceed 10 - 9. If there is no such x then output  - 1 as the answer.

Examples

Input

3 1


Output

1.000000000000


Input

1 3


Output

-1


Input

4 1


Output

1.250000000000

Note

You can see following graphs for sample 1 and sample 3. 

<image> <image>
/*
 * Remember a 6.89 student can know more than a 10.0 student.
 * Grades don't determine intelligence, they test obedience. 
 * I Never Give Up.
 * I will become Candidate Master today.
 * I will defeat Saurabh Anand.
 * Skills are Cheap,Passion is Priceless.
 */
import java.util.*;
import java.util.Map.Entry;
import java.io.*;

import static java.lang.System.out;
import static java.util.Arrays.*;
import static java.lang.Math.*;
 
public class ContestMain{
	private static Reader in=new Reader();
	private static StringBuilder ans=new StringBuilder();
	private static long MOD=(long)1e9+7;
	private static long INF=(long)1e12;
	private static final int N=(int) (1e5+7); // 1e5+7
    private static final double EPS=1e-9;
    private static final int LIM=26;
    private static final double PI=3.1415;
//    private static ArrayList<Integer> v[]=new ArrayList[N];
//    private static int color[]=new int[N];
	private static boolean mark[]=new boolean[N];
//    private static BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    private static ArrayList<Pair> v[]=new ArrayList[N];
    private static ArrayList<Pair> vrev[]=new ArrayList[N];
	private static long powmod(long x,long n){
		if(n==0||x==0)
			return 1;
		else if(n%2==0)
			return(powmod((x*x)%MOD,n/2));
		else
			return (x*(powmod((x*x)%MOD,(n-1)/2)))%MOD;
	}
	private static void shuffle(long[] arr){
		for(int i=arr.length-1;i>=2;i--){
			int x=new Random().nextInt(i-1);
			long temp=arr[x];
			arr[x]=arr[i];
			arr[i]=temp;
		}
	}
	private static void shuffle(int[] arr){
		for(int i=arr.length-1;i>=2;i--){
			int x=new Random().nextInt(i-1);
			int temp=arr[x];
			arr[x]=arr[i];
			arr[i]=temp;
		}
	}
    private static long gcd(long a, long b){
        if(b==0)return a;
        return gcd(b,a%b);
    }
//    private static boolean check(int x,int y){
//        if((x>=0&&x<n)&&(y>=0&&y<m)&&mat[x][y]!='X'&&!visited[x][y])return true;
//        return false;
//    }
    public static int mod(int n,int c){
    	if(n<0)return n%c+c;
    	return n%c;
    }
	public static void main(String[] args) throws IOException{
		double a=in.nextLong();
		double b=in.nextLong();
		if(b>a)pn(-1);
		else if(a==b)pf("%.9f",a);
		else{
			pf("%.9f",min((a-b)/(2*(floor((a-b)/(2*b*1.0)))),(a+b)/(2*(floor((a+b)/(2*b*1.0))))));
		}
	}
	static class Pair<T> implements Comparable<Pair>{
		int l;
		long r;
		Pair(){
			l=0;
			r=0;
		}
		Pair(int k,long v){
			l=k;
			r=v;
		}
		@Override
		public int compareTo(Pair o){
			return (int)(r-o.r);
		}
		// Fenwick tree question comparator
//        @Override
//        public int compareTo(Pair o) {
//            if(o.r!=r)return (int) (r-o.r);
//            else return (int)(l-o.l);
//        }
	}
	//Reader Class
	static class Reader{
		BufferedReader br;
		StringTokenizer st;
		public Reader(){br=new BufferedReader(new InputStreamReader(System.in));}
		String next(){
			while(st==null||!st.hasMoreElements()){
				try{st=new StringTokenizer(br.readLine());
				}catch(IOException e){e.printStackTrace();}
			}
			return st.nextToken();
		}
		int nextInt(){return Integer.parseInt(next());}
		long nextLong(){return Long.parseLong(next());}
		double nextDouble(){return Double.parseDouble(next());}
		String nextLine(){
			String str="";
			try{
				str=br.readLine();
			}catch(IOException e){
				e.printStackTrace();
			}
			return str;
		}
	}
	//Printer Methods
	static void pn(Object o){out.print(o);}
	static void pln(Object o){out.println(o);}
	static void pln(){out.println();}
	static void pf(String format,Object o){out.printf(format,o);}
	//Appenders
	static void app(Object o){ans.append(o);}
}