n = int(input())

if n == 2:
    print(1)
    print(1, 2)
elif n == 3:
    print(2)
    print(1, 3)
    print(2, 3)
else:
    edges = set()
    # Connect vertex 1 to all others
    for j in range(2, n + 1):
        edges.add((1, j))
    # Connect vertex 2 to 3,4,...,n-1
    for j in range(3, n):
        edges.add((2, j))
    # Connect 3,4,...,n-1 to vertex 2
    for j in range(3, n):
        edges.add((j, 2))
    # Connect vertex n to 1
    edges.add((1, n))
    # Convert to list and remove duplicates
    edges = list(edges)
    m = len(edges)
    print(m)
    for u, v in edges:
        print(u, v)