n, k = map(int, input().split())
if n == 1:
    if k == 0:
        print(-1)
    else:
        print(1)
        print(1, 1)
    exit()

if 2 * k < n:
    print(-1)
    exit()

print(n + k)
for i in range(1, k + 1):
    print(i, i)
j = 1 + n // 2
for i in range(1, n + 1):
    print(i, j)
    j += 1
    if j > n:
        j -= n