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 
#!/usr/bin/env python
# -*- coding:utf-8 -*-
#

def readint(): return int(raw_input())
def readfloat(): return float(raw_input())
def readarray(N, foo):
        res = []
        for _ in xrange(N):
                res.append(foo())
        return res
def readlinearray(foo): return map(foo, raw_input().split())

class Node:
    
    def __init__(self, id):
        self.neighbours = []
        self.id = id
        
    def add(self, neighbour):
        self.neighbours.append(neighbour)

def visit(node, visited, redges):
    if visited[node.id]: return
    visited[node.id] = True
    for neighbour in node.neighbours:
        if not visited[neighbour.id]:
            visit(neighbour, visited, redges)
#            visited[neighbour.id] = True
        else:
            if node.id < neighbour.id:
                redges.add((node.id, neighbour.id))
            else:
                redges.add((neighbour.id, node.id))
    

def get(N, roads, nodes):
    visited = (N+1)*[False]
    
    ts = []
    redges = set() #repeated edges
    first_node = None
    for node in nodes:
        if  not visited[node.id]:
            if redges:
                ts.append(redges.pop() + (first_node, node.id))
                redges = set()
            first_node = node.id
            visit(node, visited, redges)
    
    return ts

def main():
    N = input()
    
    nodes = []
    for i in xrange(1, N+1):
        nodes.append(Node(i))
    
    roads = []
    for n in xrange(N-1):
        a, b = map(int, raw_input().split())
        
        nodes[a-1].add(nodes[b-1])
        nodes[b-1].add(nodes[a-1])
        
        if a > b:
            a, b = b, a
        roads.append((a, b))
    ts = get(N, roads, nodes)
    print len(ts)
    for i, j, u, v in ts:
        print i, j, u, v

main()
