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 
class DSU:
    def __init__(self, n):
        self.parent = [*range(n)]
        self.set_size = [1] * n
        self.roots = set(self.parent)

    def root(self, a):
        while a != self.parent[a]:
            a = self.parent[a]
        return a

    def join(self, a, b):
        a = self.root(a)
        b = self.root(b)

        if self.set_size[a] < self.set_size[b]:
            a, b = b, a

        if a == b:
            return False

        self.roots.remove(b)
        self.set_size[a] += self.set_size[b]
        self.set_size[b] = 0
        self.parent[b] = a

        return True


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

d = DSU(n)
for _ in range(n - 1):
    i = [*map(int, input().split())]
    if d.join(i[0] - 1, i[1] - 1) is False:
        roads.append(i)

ans = []

while len(d.roots) > 1 and len(roads):
    i = iter(d.roots)
    ans.append((roads[-1][0], roads[-1][1], next(i) + 1, next(i) + 1))
    d.join(ans[-1][2] - 1, ans[-1][3] - 1)
    del roads[-1]

print(len(ans))
for i in ans:
    print(*i)
