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 
# -*- coding:utf-8 -*-

"""

created by shuangquan.huang at 1/14/20

"""

import collections
import time
import os
import sys
import bisect
import heapq
from typing import List


def solve(N, edges):
    
    dsu = collections.defaultdict(int)
    
    def find(u):
        if dsu[u] == 0:
            return u
        
        p = find(dsu[u])
        dsu[u] = p
        return p
    
    def union(u, v):
        pu, pv = find(u), find(v)
        dsu[pu] = pv
        
    def same(u, v):
        return find(u) == find(v)
    
    remove = []
    for u, v in edges:
        if same(u, v):
            remove.append((u, v))
        else:
            union(u, v)
    groups = list(sorted({find(u) for u in range(1, N+1)}))
    added = []
    for u, v in zip(groups[:-1], groups[1:]):
        added.append((u, v))
    
    if not remove:
        print(0)
    else:
        print(len(remove))
        for i in range(len(remove)):
            print('{} {} {} {}'.format(remove[i][0], remove[i][1], added[i][0], added[i][1]))
    

N = int(input())
edges = []
for i in range(N-1):
    u, v = map(int, input().split())
    edges.append((u, v))
solve(N, edges)