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()
    # Add star edges from node 1 to all others
    for v in range(2, n + 1):
        edges.add((1, v))
    # Add edges from node 2 to nodes 3,4,...,n-1
    for v in range(3, n):
        edges.add((2, v))
    # Output the edges
    print(len(edges))
    for u, v in sorted(edges):
        print(u, v)