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>).
# problem statement: https://codeforces.com/problemset/problem/578/A

coord = input()
a, b = list(map(int, coord.split()))

if b > a:
	print(-1)
else:
	print((a + b)/(2 * int((a+b)/2)))
