N=int(input())
E=[]
S=list(range(N))
S[-1]-=1
for i in range(N):
  for j in range(S[i]):
    S[-j-1]-=1
    if i+1==N-j:
      break
    E.append((i+1,N-j))
print(len(E))
for i in range(len(E)):
  print(*E[i])