n, k = map(int, input().split())
if n > 2 * k:
    print(-1)
    exit()

ANS = []
for i in range(1, n - k + 1):
    p = 2 * i - 1
    q = 2 * i
    ANS.append((p, p))
    ANS.append((p, q))
    ANS.append((q, p))
for i in range(2 * (n - k) + 1, n + 1):
    ANS.append((i, i))
print(len(ANS))
for x, y in ANS:
    print(x, y)