import subprocess
import random
import time

random.seed(23)


def make_tree(l, r=None):
    if r is None:
        n = l
    else:
        n = random.randrange(l, r)
    t = random.random()
    edges = []
    if t < 0.1:  # path
        for i in range(n - 1):
            if random.random() < 0.5:
                edges.append((i, i + 1))
            else:
                edges.append((i + 1, i))
    elif t < 0.2:  # star
        for i in range(1, n):
            if random.random() < 0.5:
                edges.append((0, i))
            else:
                edges.append((i, 0))
    else:  # other
        for i in range(1, n):
            j = random.randrange(i)
            if random.random() < 0.5:
                edges.append((i, j))
            else:
                edges.append((j, i))

    P = list(range(1, n + 1))
    random.shuffle(P)
    for i, (u, v) in enumerate(edges):
        edges[i] = (P[u], P[v])
    return n, edges


class UnionFind:
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        self.group = n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return
        self.group -= 1
        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return self.group

    def all_group_members(self):
        dic = {r: [] for r in self.roots()}
        for i in range(self.n):
            dic[self.find(i)].append(i)
        return dic

    def __str__(self):
        return "\n".join("{}: {}".format(r, self.members(r)) for r in self.roots())


random.seed(3)
n, edges = make_tree(200000)
print(n)
for u, v in edges:
    print(u, v)

# T = 10000
# lst = []
# for _ in range(T):
#     # subprocess の処理は結構時間がかかるので確認用
#     if _ % 100 == 0:
#         print("!", _)
#         pass

#     random.seed(_)
#     # テストケース作成
#     n, edges = make_tree(200000)

#     inp = []
#     inp.append(f"{n}")
#     for u, v in edges:
#         inp.append(f"{u} {v}")
#     inp = "\n".join(inp)
#     # print(inp)
#     # print()

#     # A.py, B.py に提出用の解と愚直解を書いて答えを比較
#     res1 = subprocess.run(f"./a.out", input=inp, text=True, shell=True, capture_output=True)
#     res2 = subprocess.run(f"./B.out", input=inp, text=True, shell=True, capture_output=True)

#     # もし解が違ったら in にテストケースが入ってる
#     if res1.stdout != res2.stdout:
#         print("!", _)
#         # print(inp)
#         with open("in", "w") as f:
#             f.write(inp)
#         assert False, (res1.stdout, res2.stdout)