var f,g:text;
    n,i:longint;
begin
assign(f,'dame.in');
assign(g,'dame.out');
reset(f);
rewrite(g);
read(f,n);
if (n=1)then
  begin
    writeln(g,1);
    writeln(G,1,' ',1);
  end else
if (n=2)then
  begin
    writeln(G,1);
    writeln(G,1,' ',1);
  end else
if (n=3)then
  begin
    writeln(g,2);
    writeln(G,1,' ',1);
    writeln(g,3,' ',2);
  end else
begin
  writeln(g,n);
  i:=1;
  while (i<=n)do
    begin
      writeln(g,i,' ',i div 2+1);
      inc(i,2);
    end;
  i:=2;
  while (i<=n)do
    begin
      writeln(g,i,' ',(n-1) div 2+1+(i div 2));
      inc(i,2);
    end;
end;
close(f);
close(g);
end.