defmodule Main do
  def input, do: IO.read(:line) |> String.trim
  def ii, do: input() |> String.to_integer
  def li, do: input() |> String.split |> Enum.map(&String.to_integer/1)
  def yn(b), do: IO.puts(if b, do: "Yes", else: "No")

  def main do
    # _n = ii()
    IO.binread(:line)
    a = IO.binread(:line) |> String.trim |> String.split |> Enum.map(&String.to_integer/1)

    # gg = a |> group_by2(fn a, b -> a+1 == b end)
    # for g <- gg, reduce: 0 do
    #   acc ->
    #     k = length(g)
    #     div(k * (k-1), 2) + acc
    # end
    # |> IO.puts

    gg = a |> gp2(fn a, b -> a+1 == b end)
    IO.puts gg
  end

  def group_by2([], _pred), do: []
  def group_by2(xs, pred) do
    n = Stream.chunk_every(xs, 2, 1, :discard)
    |> Stream.take_while(fn [a, b] -> pred.(a, b) end)
    |> Enum.count

    {a, b} = Enum.split(xs, n+1)
    [a | group_by2(b, pred)]
  end

  def gp2([], _pred), do: 0
  def gp2(xs, pred) do
    n = Stream.chunk_every(xs, 2, 1, :discard)
    |> Stream.take_while(fn [a, b] -> pred.(a, b) end)
    |> Enum.count

    b = Enum.drop(xs, n+1)
    k = div((n+1) * n, 2)
    k + gp2(b, pred)
    #[n+1 | gp2(b, pred)]
  end
end