parseInt(x)=parse(Int,x)
parseMap(x::Array{SubString{String},1})=map(parseInt,x)

function main()
	a,b = readline() |> split |> parseMap
	c = Int[]
	r = 1
	while r <= sqrt(a+b)
		if (a+b)%r == 0
			push!(c,r)
			push!(c,div(a+b,r))
		end
		r += 1
	end
	c = sort(c)
	f = 0
	for i in 1:length(c)
		if (a+c[i])%b==0 && (b+c[i])%a==0 && a!=c[i] && b!=c[i]
			println(c[i])
			f = 1
			break
		end
	end
	if f == 0
		println(-1)
	end
end
main()