defmodule Main do

    def is_answer(a, b, _) when (a == b), do: false
    def is_answer(a, _, c) when (a == c), do: false
    def is_answer(_, b, c) when (b == c), do: false
    def is_answer(a, b, c) do
        r1 = rem(a + b, c)
        r2 = rem(a + c, b)
        r3 = rem(b + c, a)
        r1 == 0 && r2 == 0 && r3 == 0
    end

    def solve(a, b) do
        r = rem(a + b, 2)
        d2 = div(a + b, 2)
        cond do
            is_answer(a, b, a + b) -> (a + b)
            r == 0 && is_answer(a, b, d2) -> d2
            true -> -1
        end
    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