program Project1;
{$APPTYPE CONSOLE}
uses
  SysUtils;
var n,i,j,ans: longint;
    inp,outp: text;
    a: array[1..10000000]of longint;
    b: array[1..20000000]of integer;
function NSD(var m,b: longint): longint;
label 1;
var l,k: integer;
    c: longint;
begin
   k:=m;
   l:=b;
   if k=l then
      begin
         NSD:=k;
         goto 1;
      end;
   while (k<>0)and(l<>0)do
      if k>l then
         k:=k-l
      else
         l:=l-k;
   if k<>0 then NSD:=k
   else
      NSD:=l;
1:
end;
begin
   assign(inp,'pair.in');
   assign(outp,'pair.out');
   reset(inp);
   rewrite(outp);
   readln(inp,n);
   ans:=0;
   for i:=1 to n do
      a[i]:=i;
   for i:=1 to n do
      for j:=1 to n do
         if a[i]=a[j]then inc(ans) else
         if (a[i]=NSD(a[i],a[j]))and(b[i+j]<>1)then
            begin
               inc(ans);
               writeln(outp,i,' ',j);
               b[i+j]:=1;
            end;
   if n=1 then writeln(outp,'1')
   else
      writeln(outp,ans);

   close(inp);
   close(outp);
end.