var a:array[1..1010]of integer;
    n,i,j,z,q,x:integer;
begin
     assign(input,'dame.in');
     reset(input);
     assign(output,'dame.out');
     rewrite(output);
     readln(n);
     q:=n mod 2;
     x:=n+2;
     for i:=1 to (n+1)div 2 do
     begin
          x:=x-2;
          a[i]:=x;
          z:=z+1;
     end;
     if q=1 then x:=n div 2 -1
     else x:=n div 2-2;
     i:=(n+1) div 2;
     while x<n-1 do
     begin
          x:=x+2;
          i:=i+1;
          a[i]:=x;
          z:=z+1;
     end;
     if q=1 then x:=0
            else x:=-1;
     while i<n do
     begin
          x:=x+2;
          i:=i+1;
          a[i]:=x;
          z:=z+1;
     end;
     writeln(z);
     for i:=1 to z do
     begin
          writeln(a[i],' ',i);
     end;
close(output);
end.