a, b=[int(k) for k in input().split()]
if b>a:
print(-1)
else:
if (a%b)==0 and (a//b)%2==1:
print(b)
else:
c=(a//b-1)//2*2+1
x=(a+b)/(1+c)
print(x)
1676G - White-Black Balanced Subtrees | 1716D - Chip Move |
1352F - Binary String Reconstruction | 1487B - Cat Cycle |
1679C - Rooks Defenders | 56A - Bar |
1694B - Paranoid String | 35A - Shell Game |
1684A - Digit Minimization | 43B - Letter |
1017A - The Rank | 1698B - Rising Sand |
235A - LCM Challenge | 1075B - Taxi drivers and Lyft |
1562A - The Miracle and the Sleeper | 1216A - Prefixes |
1490C - Sum of Cubes | 868A - Bark to Unlock |
873B - Balanced Substring | 1401D - Maximum Distributed Tree |
1716C - Robot in a Hallway | 1688B - Patchouli's Magical Talisman |
99A - Help Far Away Kingdom | 622B - The Time |
1688C - Manipulating History | 1169D - Good Triple |
1675B - Make It Increasing | 588A - Duff and Meat |
1541B - Pleasant Pairs | 1626B - Minor Reduction |