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 
n=int(input())
# Python3 program to implement Disjoint Set Data
# Structure.

class DisjSet:
    def __init__(self, n):
        # Constructor to create and
        # initialize sets of n items
        self.rank = [1] * n
        self.parent = [i for i in range(n)]
        self.extra=[]

    # Finds set of given item x
    def find(self, x):

        # Finds the representative of the set
        # that x is an element of
        if (self.parent[x] != x):
            # if x is not the parent of itself
            # Then x is not the representative of
            # its set,
            self.parent[x] = self.find(self.parent[x])

        # so we recursively call Find on its parent
        # and move i's node directly under the
        # representative of this set

        return self.parent[x]

    # Do union of two sets represented
    # by x and y.
    def Union(self, x, y):

        # Find current sets of x and y
        xset = self.find(x)
        yset = self.find(y)

        # If they are already in same set
        if xset == yset:
            self.extra.append((x,y))
            return

        # Put smaller ranked item under
        # bigger ranked item if ranks are
        # different
        if self.rank[xset] < self.rank[yset]:
            self.parent[xset] = yset

        elif self.rank[xset] > self.rank[yset]:
            self.parent[yset] = xset

        # If ranks are same, then move y under
        # x (doesn't matter which one goes where)
        # and increment rank of x's tree
        else:
            self.parent[yset] = xset
            self.rank[xset] = self.rank[xset] + 1


# Driver code
obj = DisjSet(n)
for _ in range(n-1):
    a,b=list(map(lambda x:int(x),input().split()))
    obj.Union(a-1,b-1)
rep=obj.find(0)
add=[]
for i in range(1,n):
    rep_1=obj.find(i)
    if rep_1!=rep:
        obj.Union(0,i)
        add.append(i)
print(len(add))
for i in range(0,len(add)):
    a,b=obj.extra[i]
    print(a+1,b+1,0+1,add[i]+1)


