n = 13
e = [
    (1, 2),
    (2, 3),
    (2, 4),
    (1, 5),
    (5, 6),
    (5, 7),
    (1, 8),
    (8, 9),
    (8, 10),
    (1, 11),
    (11, 12),
    (12, 13)
]
print(n)
for u, v in e:
    print(u, v)