There is a polyline going through points (0, 0) – (*x*, *x*) – (2*x*, 0) – (3*x*, *x*) – (4*x*, 0) – ... - (2*kx*, 0) – (2*kx* + *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*.
Only one line containing two positive integers *a* and *b* (1 ≤ *a*, *b* ≤ 10<sup class="upper-index">9</sup>).

import math
from decimal import Decimal
import sys

inp=input().split()
a=int(inp[0])
b=int(inp[1])
if b>a:
    print(-1)
else:
        s=a+b
        d=a-b
        c1=int((a+b)/b)
        if c1%2==1:
            c1-=1
        c2=int((a-b)/b)
        if c2%2==1:
            c2-=1
        a1=Decimal(s)/Decimal(c1)
        a2=Decimal(d)/Decimal(c2)
        ans=min(a1,a2)
        print(ans)
