defmodule Main do

    def divisors(n, curr) when curr > n, do: []
    def divisors(n, curr) when rem(n, curr) == 0, do: [curr, div(n, curr)] ++ divisors(n, curr + 1)
    def divisors(n, curr), do: divisors(n, curr + 1)

    def find([], _, _), do: -1
    def find([x|xs], a, b) do
        cond do
            rem(x + a, b) == 0 && rem(x + b, a) == 0 && a != x && b != x -> x
            true -> find(xs, a, b)
        end
    end

    def solve(a, b) do
        divisors(a + b, 1) |> find(a, b)
    end

    def main do
        [a, b] = IO.gets("") |> String.trim |> String.split |> Enum.map(&String.to_integer/1)
        solve(a, b) |> IO.puts
    end
end