N = int(input())
edge = []
deg = [0] * (N+1)

now = N
for d in range(1, N)[::-1]:
    add = d - deg[now]
    if add <= 0:
        break
    deg[now] = d
    for i in range(1, add+1):
        deg[now-i] += 1
        edge.append((now, now-i))
    now -= 1

print(len(edge))
for a, b in edge:
    print(a, b)