n = int(input())
d = list(range(n))
ans = []
l = 0
r = n
while r - l > 2:
    for i in range(l + 1, r - 1):
        ans.append((l, i))
    l += 1
    r -= 1
print(len(ans))
for u, v in ans:
    print(u + 1, v + 1)