N, K = map(int, input().split())

if K < (N + 1) // 2:
    print(-1)
    exit()


if N == K:
    print(N)
    for i in range(1, N+1):
        print(i, i)
    exit()


def gen():
    yield from range(1, N, 2)
    yield from range(2, N, 2)


ans = []
g = gen()
for i in range(K):
    x = next(g)
    ans.append((x, x + 1))
for i in range(1, N+1):
    ans.append((i, i))

print(len(ans))
for a, b in ans:
    print(a, b)