import sys
n,k = map(int,input().split())#頂点、線
ans = [[i,i] for i in range(1,n+1)]
if n // 2 + n % 2 < k:
  print(-1)
  sys.exit()
else:
  c = n - k
  for i in range(c):
    ans.insert(0,[i+1,n-i])
ANS = [str(len(ans))]
for i in ans:
  ANS.append(" ".join(map(str,i)))
print("\n".join(ANS))