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 
# your code goes here





class dsu:

	def __init__ ( self , n ):

		self.p = []

		for x in range(n):

			self.p.append ( x )

	def find(self , v):

		if self.p[v] == v:

			return v

		else :

			self.p[v] = self.find ( self.p[v] )

			return self.p[v]

	def union(self,a,b):

		a = self.find ( a )

		b = self.find ( b )

		if a != b:

			self.p[a] = b

	def __str__ ( self ):

		return str ( self.p )

		





n = int (input() )

DSU = dsu ( n+1 )

WRONG = [] 

for x in range(n-1):

	ss = input().split()

	a = int ( ss[0] )

	b = int ( ss[1] )

	

	if DSU.find(a) == DSU.find(b):

		WRONG.append ( [ a , b ] )

	else:

		DSU.union ( a , b )



z = 2



print ( len ( WRONG ) )

for x in range ( len ( WRONG ) ):

	while ( DSU.find ( 1 ) == DSU.find ( z ) ):

		z+=1

	DSU.union ( 1 , z )

	print ( WRONG[x][0] , WRONG[x][1] , 1 , z )



# Made By Mostafa_Khaled