# require "graph"
struct Edge(T)
  include Comparable(Edge(T))

  property to : Int32
  property cost : T

  def initialize(@to : Int32, @cost : T)
  end

  def <=>(other : Edge(T))
    {cost, to} <=> {other.cost, other.to}
  end

  def to_s(io) : Nil
    io << '(' << to << ", " << cost << ')'
  end

  def inspect(io) : Nil
    io << "->#{to}(#{cost})"
  end
end

struct Edge2(T)
  include Comparable(Edge2(T))

  property from : Int32
  property to : Int32
  property cost : T

  def initialize(@from : Int32, @to : Int32, @cost : T)
  end

  def <=>(other : Edge2(T))
    {cost, from, to} <=> {other.cost, other.from, other.to}
  end

  def reverse
    Edge2(T).new(to, from, cost)
  end

  def to_s(io) : Nil
    io << '(' << from << ", " << to << ", " << cost << ')'
  end

  def inspect(io) : Nil
    io << "#{from}->#{to}(#{cost})"
  end
end

struct UnweightedEdge2
  property from : Int32
  property to : Int32

  def initialize(@from, @to)
  end

  def reverse
    UnweightedEdge2.new(to, from)
  end

  def to_s(io) : Nil
    io << '(' << from << ", " << to << ')'
  end

  def inspect(io) : Nil
    io << "#{from}->#{to}"
  end
end

abstract class Graph(T)
  getter graph : Array(Array(Edge(T)))

  def initialize(size : Int)
    raise ArgumentError.new("Negative graph size: #{size}") unless size >= 0
    @graph = Array.new(size) { Array(Edge(T)).new }
  end

  abstract def add_edge(edge : Edge2(T))

  def add_edge(i : Int32, j : Int32, cost : T)
    add_edge(Edge2.new(i, j, cost))
  end

  def add_edges(edges : Array(Edge2(T)))
    edges.each { |edge| add_edge(edge) }
    self
  end

  delegate size, to: @graph
  delegate :[], to: @graph

  def each_edge : Nil
    (0...size).each do |v|
      graph[v].each do |edge|
        yield Edge2(T).new(v, edge.to, edge.cost)
      end
    end
  end

  def edges
    result = [] of Edge2(T)
    each_edge do |edge|
      result << edge
    end
    result
  end
end

class DirectedGraph(T) < Graph(T)
  def initialize(size : Int)
    super
  end

  def initialize(size : Int, edges : Array(Edge2(T)))
    super(size)
    add_edges(edges)
  end

  def add_edge(edge : Edge2(T))
    raise IndexError.new unless 0 <= edge.from < size
    raise IndexError.new unless 0 <= edge.to < size
    @graph[edge.from] << Edge.new(edge.to, edge.cost)
    self
  end
end

class UndirectedGraph(T) < Graph(T)
  def initialize(size : Int)
    super
  end

  def initialize(size : Int, edges : Array(Edge2(T)))
    super(size)
    add_edges(edges)
  end

  def add_edge(edge : Edge2(T))
    raise IndexError.new unless 0 <= edge.from < size
    raise IndexError.new unless 0 <= edge.to < size
    @graph[edge.from] << Edge.new(edge.to, edge.cost)
    @graph[edge.to] << Edge.new(edge.from, edge.cost)
    self
  end
end

abstract class UnweightedGraph
  getter graph : Array(Array(Int32))

  def initialize(size : Int)
    raise ArgumentError.new("Negative graph size: #{size}") unless size >= 0
    @graph = Array.new(size) { Array(Int32).new }
  end

  abstract def add_edge(edge : UnweightedEdge2)

  def add_edge(i : Int32, j : Int32)
    add_edge(UnweightedEdge2.new(i, j))
  end

  def add_edges(edges : Array(UnweightedEdge2))
    edges.each { |edge| add_edge(edge) }
    self
  end

  delegate size, to: @graph
  delegate :[], to: @graph

  def each_edge : Nil
    (0...size).each do |v|
      graph[v].each do |u|
        yield UnweightedEdge2.new(v, u)
      end
    end
  end

  def edges
    result = [] of UnweightedEdge2
    each_edge do |edge|
      result << edge
    end
    result
  end
end

class UnweightedDirectedGraph < UnweightedGraph
  def initialize(size : Int)
    super
  end

  def initialize(size : Int, edges : Array(UnweightedEdge2))
    super(size)
    add_edges(edges)
  end

  def add_edge(edge : UnweightedEdge2)
    raise IndexError.new unless 0 <= edge.from < size
    raise IndexError.new unless 0 <= edge.to < size
    @graph[edge.from] << edge.to
    self
  end
end

class UnweightedUndirectedGraph < UnweightedGraph
  def initialize(size : Int)
    super
  end

  def initialize(size : Int, edges : Array(UnweightedEdge2))
    super(size)
    add_edges(edges)
  end

  def add_edge(edge : UnweightedEdge2)
    raise IndexError.new unless 0 <= edge.from < size
    raise IndexError.new unless 0 <= edge.to < size
    @graph[edge.from] << edge.to
    @graph[edge.to] << edge.from
    self
  end
end

n = read_line.to_i
g = UnweightedUndirectedGraph.new(n)
(1...n).reverse_each do |i|
  v = 0
  while g[i].size < i
    g.add_edge(i, v)
    v = v == 0 ? i - 1 : v - 1
  end
end
puts g.edges.size // 2
puts g.edges.select { |e| e.from < e.to }.join('\n') { |e|
  "#{e.from + 1} #{e.to + 1}"
}