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:
	nodes = []
	duplicates = []

	def __init__(self,n):
		self.nodes = [[0,0] for i in range(n)]

	def find(self,x):
		return self.nodes[x]

	def union(self,x,y):
		xRoot,yRoot = self.find(x),self.find(y)
		if xRoot[0] == yRoot[0]:
			self.duplicates.append([x,y])
			return
		connections = xRoot[1] + yRoot[1]
		xRoot_pointer__original = xRoot[0]
		yRoot_pointer__original = yRoot[0]
		if xRoot[1] > yRoot[1]:
			for node in self.nodes:
				if node[0] == yRoot_pointer__original or node[0] == xRoot_pointer__original:
					node[0] = xRoot_pointer__original
					node[1] = connections
		else:
			for node in self.nodes:
				if node[0] == xRoot_pointer__original or node[0] == yRoot_pointer__original:
					node[0] = yRoot_pointer__original
					node[1] = connections

	def insert(self,x,y):
		if self.nodes[x][0] == 0 and self.nodes[y][0] == 0:
			self.nodes[x] = [x,2]
			self.nodes[y] = [x,2]
		elif self.nodes[x][0] != 0 and self.nodes[y][0] != 0:
			self.union(x,y)
		elif self.nodes[x][0] != 0:
			self.nodes[y] = [y,1]
			self.union(x,y)
		elif self.nodes[y][0] != 0:
			self.nodes[x] = [x,1]
			self.union(x,y)

n = int(input())
dsu = DSU(n+1)
for i in range(n-1):
	a = list(map(int,input().split()))
	x,y = a[0],a[1]
	dsu.insert(x,y)

new_roads = []
not_connected = 0
connected = -1

for i in range(1,len(dsu.nodes)):
	if dsu.nodes[i] != [0,0]:
		connected = i
		break

for i in range(0,n):
	if dsu.nodes[i] == [0,0]:
		not_connected += 1



for i in range(len(dsu.duplicates)):
	for i in range(1,len(dsu.nodes)):
		if dsu.nodes[i][0] != connected and dsu.nodes[i] != [0,0]:
			dsu.union(connected,i)
			new_roads.append([connected,i])
			break




for i in range(1,len(dsu.nodes)):
	if dsu.nodes[i] == [0,0]:
		#dsu.union(connected,i)
		new_roads.append([connected,i])

print(len(new_roads))

for i in range(len(new_roads)):
	print(dsu.duplicates[i][0],dsu.duplicates[i][1],new_roads[i][0],new_roads[i][1])



'''
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
'''