var f,g:text;
    n,i,j:longint;
begin
assign(f,'dame.in');reset(f);
assign(g,'dame.out');rewrite(g);
readln(f,n);
if (n=2)or(n=3)then writeln(g,n-1)
else writeln(g,n);
if n mod 2=0 then begin
                  i:=1;
                  j:=n div 2;
                  while (i<=n)and(j>=1) do
                        begin
                        writeln(g,i,' ',j);
                        i:=i+2;
                        j:=j-1;
                        end;
                  i:=n;
                  j:=n div 2+1;
                  while (i>=1)and(j<=n) do
                        begin
                        writeln(g,i,' ',j);
                        i:=i-2;
                        j:=j+1;
                        end;
                  end
else begin
     i:=1;
     j:=n div 2+1;
     while (i<=n)and(j>=1) do
                        begin
                        writeln(g,i,' ',j);
                        i:=i+2;
                        j:=j-1;
                        end;
     i:=n-1;
     j:=n div 2+2;
     while (i>=1)and(j<=n) do
                  begin
                        writeln(g,i,' ',j);
                        i:=i-2;
                        j:=j+1;
                  end;
     end;
close(g);
end.