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

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

2.upto(N - 2) do |u|
  edges << [u, u + 1]
end

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