n, k = map(int, input().split())
r = n - k
if r > n // 2:
    print(-1)
    exit()
es = []
for i in range(r):
    es.append((2 * i, 2 * i + 1))
for i in range(n):
    es.append((i, i))
print(len(es))
for u, v in es:
    print(u + 1, v + 1)