578A - A Problem about Polyline - CodeForces Solution

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)