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

C = [[False, False] for _ in range(n)]
ANS = []
for i in range(k):
    ANS.append((i + 1, i + 1))
    C[i][0] = C[i][1] = True
for i in range(n):
    if not C[i][0]:
        ANS.append((i + 1, 1))
    if not C[i][1]:
        ANS.append((1, i + 1))
print(len(ANS))
for x, y in ANS:
    print(x, y)