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=map(int,input().split())
print(-1) if a<b else print(a) if a==b else print(min(10**18 if a//b==1 else (a-b)/(2*((a//b-1)//2)),(a+b)/(2+2*((a//b-1)//2))))