mod = 998244353


def main():
    import sys
    input = sys.stdin.readline

    N, K = map(int, input().split())
    if K * 2 < N:
        print(-1)
        exit()

    ans = []
    ans2 = []
    cnt = N
    i = 1
    for k in range(K, 0, -1):
        if cnt == k:
            ans.append((i, i))
            cnt -= 1
            i += 1
        else:
            ans.append((i, i + 1))
            ans2.append((i, i))
            ans2.append((i+1, i+1))
            cnt -= 2
            i += 2
    print(len(ans) + len(ans2))
    for i, j in ans:
        print(i, j)
    for i, j in ans2:
        print(i, j)


if __name__ == '__main__':
    main()