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;
const int M = 1000 + 5;
vector<int> edge[M];
int rep_node[M];
vector<int> repete_edge;
bool visited[M];
char flag[M][M];
void DFS(int node, int fathernode) {
  for (int i = 0; i < edge[node].size(); i++) {
    int child_node = edge[node][i];
    if (visited[child_node] == false) {
      visited[child_node] = true;
      DFS(child_node, node);
    } else {
      if (child_node != fathernode && flag[child_node][node] == 'n') {
        repete_edge.push_back(child_node);
        repete_edge.push_back(node);
        flag[child_node][node] = 'v';
        flag[node][child_node] = 'v';
      }
    }
  }
  return;
}
int main() {
  for (int i = 0; i < M; i++) {
    visited[i] = false;
    for (int j = 0; j < M; j++) {
      flag[i][j] = 'n';
    }
  }
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    edge[a].push_back(b);
    edge[b].push_back(a);
  }
  int current_subcomponent = 0;
  int current_visited_nodes = 0;
  int num_repete_edge = 0;
  bool allvisited = false;
  while (allvisited == false) {
    int current_node = -1;
    for (int i = 1; i <= n; i++) {
      if (visited[i] == false) {
        current_node = i;
        break;
      }
    }
    visited[current_node] = true;
    rep_node[current_subcomponent++] = current_node;
    DFS(current_node, -1);
    allvisited = true;
    for (int i = 1; i <= n; i++) {
      if (visited[i] == false) {
        allvisited = false;
        break;
      }
    }
  }
  if (current_subcomponent == 1) {
    cout << "0" << endl;
  } else {
    cout << current_subcomponent - 1 << endl;
    int num_repete_edge = 0;
    for (int i = 1; i < current_subcomponent; i++) {
      cout << repete_edge[num_repete_edge] << " "
           << repete_edge[num_repete_edge + 1] << " ";
      num_repete_edge = num_repete_edge + 2;
      cout << rep_node[i - 1] << " " << rep_node[i] << endl;
    }
  }
  return 0;
}
