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;
void unionSet(int arr[], int a, int b);
int getParent(int arr[], int a);
int main() {
  int numOfCities;
  cin >> numOfCities;
  int canBeStopped = 0;
  int numOfNodes = 0;
  int arr[numOfCities];
  vector<int> loopedRoads;
  vector<int> nodes;
  for (int i = 0; i < numOfCities; i++) {
    arr[i] = -1;
  }
  for (int i = 1; i < numOfCities; i++) {
    int city1, city2;
    cin >> city1 >> city2;
    int parent1 = getParent(arr, city1 - 1);
    int parent2 = getParent(arr, city2 - 1);
    if (parent1 == parent2) {
      canBeStopped++;
      loopedRoads.push_back(city1);
      loopedRoads.push_back(city2);
    } else {
      unionSet(arr, parent1, parent2);
    }
  }
  for (int index = 0; index < numOfCities; index++) {
    if (arr[index] < 0) {
      numOfNodes++;
      nodes.push_back(index + 1);
    }
  }
  if (numOfNodes == 1) {
    cout << 0;
  } else {
    cout << numOfNodes - 1 << endl;
    int a = nodes.back();
    nodes.pop_back();
    int b = nodes.back();
    nodes.pop_back();
    cout << loopedRoads.at((2 * 1 - 2)) << ' ' << loopedRoads.at((2 * 1 - 1))
         << ' ' << a << ' ' << b;
    for (int i = 2; i < numOfNodes; i++) {
      a = nodes.back();
      nodes.pop_back();
      b = nodes.back();
      nodes.pop_back();
      cout << endl
           << loopedRoads.at((2 * i - 2)) << ' ' << loopedRoads.at((2 * i - 1))
           << ' ' << a << ' ' << b << endl;
    }
  }
}
void unionSet(int arr[], int a, int b) {
  if (abs(arr[a]) >= abs(arr[b])) {
    int parent = getParent(arr, a);
    arr[parent] += arr[b];
    arr[b] = parent + 1;
  } else {
    int parent = getParent(arr, b);
    arr[parent] += arr[a];
    arr[a] = parent + 1;
  }
}
int getParent(int arr[], int a) {
  if (arr[a] < 0) {
    return a;
  }
  getParent(arr, arr[a] - 1);
}
