N = gets.to_i
cur = 1
edges = []

1.upto(N - 1) do |u|
  next if u == 2

  (u + 1).upto(N) do |v|
    edges << [u, v]
  end
end

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