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 
def find(u):
    if parent[u] == u:
        return u

    parent[u] = find(parent[u])
    return parent[u]


def union(a, b):
    u = parent[a]
    v = parent[b]

    if size[u] > size[v]:
        parent[v] = u
        size[u] += size[v]

    else:
        parent[u] = v
        size[v] += size[u]



n=int(input())
parent=[i for i in range(n+1)]
size = [1 for i in range(n + 1)]

torm=[]
for j in range(n-1):
    u,v=map(int,input().split())
    if find(u)!=find(v):
        union(u,v)
    else:
        torm.append([u,v])

toadd=[]
for i in range(1,n+1):
    if find(1)!=find(i):
        union(1,i)
        toadd.append([1,i])

ans=[]
for j in range(len(toadd)):
    ans.append(torm[j]+toadd[j])

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






