n = int(input())
edges = []

if n == 2:
    edges.append((1, 2))
elif n == 3:
    edges.append((1, 3))
    edges.append((2, 3))
else:
    # Connect vertex 1 to all others
    for v in range(2, n + 1):
        edges.append((1, v))
    # Connect vertex 2 to vertices 3, 4, ..., n-1
    for v in range(3, n):
        edges.append((2, v))

print(len(edges))
for u, v in edges:
    print(u, v)