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 an(x):
    for i in x:
        return i
    return None

n = int(input())
# 1 2 .. n
r = dict()
for i in range(1, n+1):
    r[i] = set()
edge = set()
for i in range(n-1):
    (a, b) = map(int, input().split())
    edge.add((min(a,b),max(a,b)))
    r[a].add(b)
    r[b].add(a)

total = set(range(1, n+1))
v = []
while len(total) > 0:
    v.append(set())
    x = total.pop()
    v[-1].add(x)
    sosed = set()
    sosed.add(x)
    while len(sosed) > 0:
        sosed2 = set()
        for i in sosed:
            for j in r[i]:
                if j in total:
                    sosed2.add(j)
                    v[-1].add(j)
                    total.discard(j)
        sosed = sosed2.copy()


need = set()
for z in v:
    if len(z) == 1:
        continue
    x = an(z)
    sosed = set()
    sosed.add(x)
    pos = set()
    pos.add(x)
    while len(sosed) > 0:
        sosed2 = set()
        for i in sosed:
            for j in r[i]:
                if j not in pos:
                    need.add((min(i,j),max(i,j)))
                    sosed2.add(j)
                    pos.add(j)

        sosed = sosed2.copy()

print(len(v)-1)
edge -= need
edge = list(edge)
for i in range(len(v)-1):
    print(edge[i][0],edge[i][1],an(v[i]), an(v[i+1]))