#int(input())
#map(int, input().split())
#list(map(int, input().split()))

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

if K * 2 < N:
    print("-1")
elif N == K:
    print(N)
    for i in range(1, N+1):
        print(i, i)
else:
    ans = []
    for i in range(1, K+1):
        ans.append((i, i))
    for i in range(1, N+1):
        ans.append((i, (i+K-1)%N+1))

    print(len(ans))
    for x in ans:
        print(" ".join([str(_) for _ in x]))