a,b=map(int,input().split())
if a==b:
    print(a)
else:
    l=2
    r=10**9-1
    while l<r:
        m=(l+r)//2
        if (a+b)/m>=b:
            l=m
        else:
            r=m+1
        if (a+b)/m>=b and (a+b)/(m+1)<=b:
            l=m
            break
    if b<=(a+b)/l and b>(a+b)/(l+1):
        print((a+b)/l)
    else:
        print(-1)