var f,g:text;
    n,i,j:integer;

procedure dame1;
begin
writeln(g,1);
write(g,'1 1');
end;

procedure dame3;
begin
writeln(g,2);
writeln(g,'1 1');
write(g,'2 3');
end;

procedure damen;
begin
writeln(g,n);
for i:=1 to n div 2 do writeln(g,i,' ',i*2);
for j:=i+1 to n do writeln(g,j,' ',(j-i)*2-1);
end;

begin
assign(f,'dame.in');
reset(f);
readln(f,n);
assign(g,'dame.out');
rewrite(g);
case n of
   1:dame1;
   2:dame1;
   3:dame3
else damen; end;
close(g);
end.