n = int(input())
if n == 2:
    print(1)
    print(1, 2)
elif n == 3:
    print(2)
    print(1, 3)
    print(2, 3)
else:
    edges = []
    # Connect vertex 1 to all others
    for j in range(2, n + 1):
        edges.append((1, j))
    # Connect other vertices as per pattern
    for i in range(2, n - 1):
        max_j = n - (i - 1)
        for j in range(i + 1, max_j + 1):
            if j > n:
                break
            edges.append((i, j))
    # Output the result
    print(len(edges))
    for u, v in edges:
        print(u, v)