4
1 4
3 1
3 4
 5
4 1
4 3
5 3
2 4
 21
7 15
13 1
14 3
4 10
2 3
16 18
17 20
16 20
8 4
3 12
2 17
13 11
16 1
13 2
13 5
8 9
6 14
3 17
16 9
13 8
 3
3 1
3 2
 6
5 2
5 3
1 4
3 1
5 6
 60
17 34
46 22
50 44
46 33
41 21
31 33
48 6
38 19
35 60
2 24
49 29
7 53
34 1
19 55
32 1
31 42
27 28
4 53
6 50
21 34
1 10
12 36
54 8
16 13
51 43
45 51
54 20
13 53
34 33
49 33
51 11
59 34
15 5
59 28
30 39
13 30
58 4
34 14
3 9
19 34
4 18
26 48
56 54
3 43
57 25
3 41
35 3
48 44
19 13
54 1
23 31
59 47
5 1
46 40
6 26
20 25
37 5
17 24
20 52
 3
3 2
1 2
 39
6 13
15 39
10 35
31 28
4 21
12 39
3 7
3 13
6 1
5 14
36 28
12 15
18 38
30 29
19 34
36 16
20 22
8 13
38 32
26 39
21 37
1 7
15 27
12 26
8 3
6 14
29 2
25 23
32 21
5 16
32 25
6 8
13 10
23 30
34 37
29 33
28 14
36 5
 10
5 9
8 5
7 6
7 9
3 9
2 1
7 2
3 6
7 1

1
3 4 1 2  0
 3
13 2 1 7 3 17 1 19 13 8 1 21  0
 0
 2
48 44 1 6 6 26 1 12  0
 7
12 15 1 2 1 7 1 9 12 26 1 11 8 3 1 12 6 8 1 17 28 14 1 20 36 5 1 24  2
3 6 1 4 7 1 1 10 
import sys
import math


def ii(): return int(input())  # input int 1


def il(): return input().split(' ')  # input list


# input list of int: [1, 2, 3, ...]
def ili(): return [int(i) for i in input().split(' ')]


# input list of int splited: (a, b, c, ...)
def ilis(): return (int(i) for i in input().split(' '))


def ip(): return input()


def op(*args, **kwargs):
    print(*args, **kwargs)
    sys.stdout.flush()


def gcd(a, b): return a if b == 0 else gcd(b, a % b)


def extgcd(a, b, x=0, y=0):
    if b == 0:
        return (a, 1, 0)
    d, m, n = extgcd(b, a % b, x, y)
    return (d, n, m-(a//b)*n)


class UFS:
    def __init__(self, n):
        self.data = [i for i in range(n)]  # 记录每个节点的祖先节点
        self.size = [1] * n               # 记录每个节点的子树的节点数
        self.cnt = n                      # 记录当前的集合数量

    def find(self, m):
        if self.data[m] == m:
            return m
        ancestor = self.find(self.data[m])  # 找到祖先节点（根节点）
        self.data[m] = ancestor
        return ancestor

    def union(self, a, b):
        ancestor_a = self.find(a)
        ancestor_b = self.find(b)
        if ancestor_a == ancestor_b:
            return
        if self.size[ancestor_a] > self.size[ancestor_b]:
            self.data[ancestor_b] = ancestor_a
            self.size[ancestor_a] += ancestor_b
        else:
            self.data[ancestor_a] = ancestor_b
            self.size[ancestor_b] += ancestor_a
        self.cnt -= 1

    def query(self, a, b):
        return self.find(a) == self.find(b)


def main():
    n = ii()
    ufs = UFS(n+1)
    dels = []
    for i in range(n-1):  # 读入边
        u, v = ilis()
        if (ufs.query(u, v)):  # 无用边，加入可删除列表
            dels.append((u, v))
        else:  # 有用边，并查集中进行合并
            ufs.union(u, v)
    roots = list(set(ufs.data))  # 各集合的祖先
    roots.pop(0)  # 从1开始编号，故将0去除
    unum = len(roots) - 1  # 待补的边个数，即：集合数-1
    if (len(dels) < unum):
        op(0)
        return
    op(unum)  # 输出最少操作数，即待补边的个数
    for num in range(unum):  # 依次从可删除边列表取边，链接各集合祖先
        op(dels[num][0], dels[num][1], roots[0], roots[num+1])


if __name__ == "__main__":
    main()
