N = gets.to_i
cur = 1
edges = [[1, 2]]
queue = [*3..N]
d = 0
r = 0
cur = 2

until queue.empty?
  v = queue.shift
  edges << [cur, v]

  if r > 0
    r -= 1
  else
    d += 1
    r = d
    cur = v
  end
end

puts edges.size
edges.each do |u, v|
  puts "%d %d" % [u, v]
end