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 
#include <bits/stdc++.h>
using namespace std;
int parent[10000];
int size[10000];
void make_set(int i) {
  parent[i] = i;
  size[i] = 1;
}
int find_set(int i) {
  if (parent[i] == i) return i;
  return parent[i] = find_set(parent[i]);
}
void union_sets(int a, int b) {
  a = find_set(a);
  b = find_set(b);
  if (a != b) {
    if (size[b] > size[a]) {
      int temp;
      temp = a;
      a = b;
      b = temp;
    }
    parent[b] = a;
    size[a] += size[b];
  }
}
int main() {
  set<int> set1;
  vector<int> vector1, vector2;
  vector<int>::iterator it1, it3;
  set<int>::iterator it2, it4;
  int test, a, b, number[10000], k = 0, size = 0;
  cin >> test;
  test = test - 1;
  int firstCity[test], secondCity[test];
  for (int i = 1; i <= test + 1; i++) {
    make_set(i);
  }
  for (int i = 0; i < test; i++) {
    cin >> firstCity[i] >> secondCity[i];
    make_set(firstCity[i]);
    make_set(secondCity[i]);
  }
  for (int i = 0; i < test; i++) {
    a = find_set(secondCity[i]);
    b = find_set(firstCity[i]);
    if (a != b) {
      union_sets(firstCity[i], secondCity[i]);
    } else {
      size++;
      vector1.push_back(firstCity[i]);
      vector2.push_back(secondCity[i]);
    }
  }
  for (int i = 1; i <= test + 1; i++) {
    int l = find_set(i);
    set1.insert(l);
  }
  cout << size << endl;
  if (size > 0) {
    auto it4 = set1.begin();
    it4++;
    for (it2 = set1.begin(), it1 = vector1.begin(), it3 = vector2.begin();
         it1 != vector1.end(); it2++, it1++, it3++, it4++) {
      cout << *it1 << " " << *it3 << " " << *it2 << " " << *it4 << " " << endl;
    }
    return 0;
  }
}
