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_set(parent,v):
    stack=[v,0]
    while stack:
        anymore=False
        x,y=stack[-2],stack[-1]
        if parent[x]!=x and y==0:
            anymore=True
            stack[-1]=1
            stack.append(parent[x])
            stack.append(0)
        elif parent[x]==x:
            dada=parent[x]
            anymore=False
        else:
            anymore=False
        if anymore==False:
            parent[x]=dada
            stack.pop()
            stack.pop()
    return parent[v]
def union_set(a,b,size,parent,brk):
    u,v=a,b
    a = find_set(parent,a)
    b = find_set(parent,b)
    if (a != b) :
        if (size[a] < size[b]):
            a,b=b,a
        parent[b] = a
        size[a] += size[b]
    else:
        brk.append([u,v])
n=int(input())
parent=[i for i in range(n+1)]
size=[1]*(n+1)
brk=[]
for i in range(n-1):
    a,b=map(int,input().split())
    union_set(a,b,size,parent,brk)
etc={}
for i in range(1,n+1):
    find_set(parent,i)
allah=parent[1]
etc[parent[1]]=1
parents=[]

for i in range(2,len(parent)):
    try:
        etc[parent[i]]+=1
    except:
        parents.append(parent[i])
        etc[parent[i]]=1
print(len(parents))
for i in range(len(parents)):
    x,y=brk.pop()
    print(x,y,allah,parents[i])