n = int(input())
edges = []

if n == 2:
    edges.append((1, 2))
elif n == 3:
    edges.append((1, 3))
    edges.append((2, 3))
else:
    # Connect central node to all
    for i in range(2, n + 1):
        edges.append((1, i))
    # Add edge 2-3
    edges.append((2, 3))
    # Add edges from 3 to N-2
    for i in range(3, n - 1):
        edges.append((i, i + 1))

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