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>).
a,b = list(map(int,input().split()))
if b > a:
    print(-1)
else:
    k = int((a+b)/2/b)
    x = (a+b)/2/k
    k = int((a-b)/2/b)
    x = min(x,(a-b)/2/k)
    print("%.9f"%x)
