defmodule Main do
    def solve(n, n, t) do
        n + t
    end
    def solve(a, b, t) do
        solve(a + 1, b, a + t)
    end

    def main do
        n = IO.gets("") |> String.trim |> String.to_integer
        b = solve(1, n, 0)
        IO.puts "#{b}"
    end
end