n = int(input())
ans = []
for i in range(n // 2):
    for j in range(i + 1, n // 2):
        ans.append((i + 1, j + 1))
for i in range(1, n // 2 + 1):
    for j in range(i):
        ans.append((n // 2 + i, j + 1))
print(len(ans))
for e in ans:
    print(*e)