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;
vector<int> a[1000];
bool used[1000][1000];
bool use[1000];
bool q = true;
void dfs(int v, int pr) {
  use[v] = true;
  for (int i = 0; i < a[v].size(); i++) {
    int to = a[v][i];
    if (q) return;
    if (!used[v][to] && use[to] && to != pr) {
      used[v][to] = true;
      used[to][v] = true;
      q = true;
      return;
    }
    if (q) return;
    if (!use[to] && !used[to][v]) dfs(to, v);
  }
}
int color[1000];
void dfs2(int v, int c) {
  use[v] = true;
  color[v] = c;
  for (int i = 0; i < a[v].size(); i++) {
    int to = a[v][i];
    if (used[to][v]) continue;
    if (!use[to]) dfs2(to, c);
  }
}
int main() {
  int n, t1, t2;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    scanf("%d %d", &t1, &t2);
    t1--;
    t2--;
    a[t1].push_back(t2);
    a[t2].push_back(t1);
  }
  memset(used, false, sizeof(used));
  memset(use, false, sizeof(use));
  bool qq = true;
  while (qq) {
    qq = false;
    for (int i = 0; i < n; i++)
      if (!use[i]) {
        dfs(i, -1);
        if (q) qq = true;
        q = false;
      }
    memset(use, false, sizeof(use));
  }
  int schet = 0;
  for (int i = 0; i < n; i++)
    if (!use[i]) {
      schet++;
      dfs2(i, schet);
    }
  vector<pair<int, int>> ans;
  for (int i = 0; i < n; i++) {
    for (int j = 1; j < n; j++)
      if (color[j] != color[0]) {
        ans.push_back(make_pair(0, j));
        int help = color[j];
        for (int z = 0; z < n; z++)
          if (color[z] == help) color[z] = color[0];
        break;
      }
  }
  vector<pair<int, int>> ans1;
  for (int i = 0; i < n; i++)
    for (int j = 0; j < n; j++) {
      if (i >= j) continue;
      if (used[i][j]) ans1.push_back(make_pair(i, j));
    }
  printf("%d\n", ans.size());
  for (int i = 0; i < ans.size(); i++)
    printf("%d %d %d %d\n", ans1[i].first + 1, ans1[i].second + 1,
           ans[i].first + 1, ans[i].second + 1);
  return 0;
}
