defmodule Main do
  def main do
    1..(IO.read(:line)
        |> String.trim()
        |> String.to_integer())
    |> Enum.map(fn _ ->
      IO.read(:line)
      |> String.trim()
      |> String.split()
      |> Enum.map(&String.to_integer/1)
    end)
    |> solve
    |> IO.puts()
  end

  def solve(x) do
    solve_rec(x, -1)
  end

  def solve_rec([], a) do
    a
  end

  def solve_rec([[x, y] | t], a) do
    cond do
      y <= x || (a != -1 && a != y - x) -> -1
      true -> solve_rec(t, y - x)
    end
  end
end