N = int(input())
if N <= 2:
  print(-1)
else: 
  M = 2 * N ** 2
  print(M)
  for i in range(1,N ** 2):
    print(f"{i} {i + 1}")
    print()
    print()
  print(f"{N ** 2}     {1}",end = "      ")
  for i in range(1,N + 1):
    for j in range(N - 1):
      print(f"{i + j * N} {i + (j + 1) * N}")
    print(f"{i + (N - 1) * N} {i}")    
  for i in range(100):
  	print()