N, K = map(int, input().split())
if 2 * K < N:
    print(-1)
    exit()
    
ans = []
for i in range(K):
    ans.append((i, i))
    
for i in range(K, N):
    ans.append((i, i - K))
    ans.append((i - K, i))
    
print(len(ans))
for a, b in ans:
    print(a + 1, b + 1)