n,k = map(int,input().split())
if 2*k < n:
	print(-1)
	exit()
else:
	ret = []
	for i in range(k):
		ret.append((i+1, n-k+i+1))
	if k != n:
		for i in range(n):
			ret.append((i+1, i+1))
	print(len(ret))
	for i,j in ret:
		print(i,j)