n = int(input())
c = [0]*(n+1)
cnt = n*(n-1)//2
cur = n
res = []
s = set()
while cnt > 0:
    tmp = cur-1-c[cur]
    for i in range(tmp):
        res.append((cur, cur-1-i))
        c[cur] += 1
        c[cur-1-i] += 1
        cnt -= 1
    for i in range(n, 0, -1):
        s.add(c[i])
    if len(s) == n-1:
        break
    cur -= 1
# print(c)
print(len(res))
for i in range(len(res)):
    a, b = res[i]
    print(n+1-a, n+1-b)