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()
    for i in range(1, n):
        for j in range(i + 1, i + (n - (i + 1)) + 1):
            edges.add((i, j))
    edges.add((n-1, n))
    print(len(edges))
    for u, v in edges:
        print(u, v)