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
from array import array  # noqa: F401


def input():
    return sys.stdin.buffer.readline().decode('utf-8')


n = int(input())
adj = [[] for _ in range(n)]
deg = [0] * n

for u, v in (map(int, input().split()) for _ in range(n - 1)):
    adj[u - 1].append(v - 1)
    adj[v - 1].append(u - 1)
    deg[u - 1] += 1
    deg[v - 1] += 1

groups = []
remove_edge = []
visited = [0] * n
visited2 = [0] * n


def dfs(stack: list):
    while stack:
        v = stack.pop()
        deg[v] = 0
        visited2[v] = 1
        for dest in adj[v]:
            if not deg[dest]:
                continue
            deg[dest] -= 1
            if deg[dest] == 1:
                stack.append(dest)


for i in range(n):
    if visited[i]:
        continue
    visited[i] = 1
    groups.append(i + 1)
    vs = [i]
    stack = [i]
    nstack = []

    while stack:
        v = stack.pop()
        if deg[v] <= 1:
            nstack.append(v)
        for dest in adj[v]:
            if not visited[dest]:
                visited[dest] = 1
                stack.append(dest)
                vs.append(dest)

    dfs(nstack)

    vs = [v for v in vs if not visited2[v]]

    while vs:
        u, v = vs[-1], [x for x in adj[vs[-1]] if not visited2[x]][0]
        adj[u].remove(v)
        adj[v].remove(u)
        deg[u] -= 1
        deg[v] -= 1
        remove_edge.append(f'{u+1} {v+1} ')
        if deg[u] == 1:
            nstack.append(u)
        if deg[v] == 1:
            nstack.append(v)

        dfs(nstack)
        vs = [v for v in vs if not visited2[v]]


ans = []
for i, s in enumerate(remove_edge):
    ans.append(s + f'{groups[i]} {groups[i+1]}')

sys.stdout.buffer.write((str(len(ans)) + '\n' + '\n'.join(ans)).encode('utf-8'))
