func main()
	var n: int :: cui@inputInt()
	var h: int :: n / 2
	if(n % 2 = 0)
		var m: int :: h * (h - 1) / 2 + h * (h - 1)
		do cui@print("\{m}\n")
		for i(0, h - 1)
			for j(0, i - 1)
				do cui@print("\{i + 1} \{n - j}\n")
			end for
		end for
		for i(h, n - 1)
			for j(i + 1, n - 1)
				do cui@print("\{i + 1} \{j + 1}\n")
			end for
		end for
	else
		var m: int :: h * (h + 1) / 2 + h * (h - 1)
		do cui@print("\{m}\n")
		for i(0, h)
			for j(0, i - 1)
				do cui@print("\{i + 1} \{n - j}\n")
			end for
		end for
		for i(h + 1, n - 1)
			for j(i + 1, n - 1)
				do cui@print("\{i + 1} \{j + 1}\n")
			end for
		end for
	end if
end func