import sys
input = sys.stdin.readline
N = int(input())
res = []
d = [0] * (N + 1)
s = {0}
for x in range(2, N + 1):
  y = x + 1
  while y <= N and d[x] in s:
    res.append((x, y))
    d[x] += 1
    d[y] += 1
  s.add(d[x])
print(len(res))
for r in res: print(*r)