n = int(input())

cnt = 0
G = [[] for _ in range(n)]
for i in range(n):
    for j in range(i + 1, n - i):
        G[i].append(j + 1)
        cnt += 1

print(cnt)
for i in range(n):
    for j in G[i]:
        print(i + 1, j)