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 i in range(2, n+1):
        edges.append((1, i))
    # Connect each vertex i (3 <= i <= n-1) to vertices 2..i-1
    for i in range(3, n):
        for j in range(2, i):
            edges.append((i, j))
    # Vertex n is connected only to 1, which is already handled

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