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> g[1005];
vector<int> comp[1005];
vector<pair<int, int>> destroy;
bool visited[1005];
bool visited2[1005];
int parent[1005];
void dfs(int j) {
  visited2[j] = true;
  stack<int> q;
  q.push(j);
  while (!q.empty()) {
    int v = q.top();
    q.pop();
    for (int i = 0; i < g[v].size(); i++) {
      int temp = g[v][i];
      if (parent[v] != temp) {
        if (!visited2[temp]) {
          visited2[temp] = true;
          parent[temp] = v;
          q.push(temp);
        } else {
          destroy.push_back(make_pair(v, temp));
        }
      }
    }
  }
}
int main() {
  int n;
  scanf("%d", &n);
  int u, v;
  int cnt = 0;
  for (int i = 0; i < n - 1; i++) {
    scanf("%d%d", &u, &v);
    g[u].push_back(v);
    g[v].push_back(u);
  }
  queue<int> q;
  for (int i = 1; i <= n; i++) {
    if (!visited[i]) {
      visited[i] = true;
      q.push(i);
      comp[cnt].push_back(i);
      while (!q.empty()) {
        v = q.front();
        q.pop();
        for (int j = 0; j < g[v].size(); j++) {
          int temp = g[v][j];
          if (!visited[temp]) {
            visited[temp] = true;
            q.push(temp);
            comp[cnt].push_back(temp);
          }
        }
      }
      cnt++;
    }
  }
  int ans = cnt - 1;
  bool good = true;
  int num = ans;
  for (int i = 1; i <= n; i++) {
    if (!visited2[i]) {
      dfs(i);
    }
  }
  printf("%d\n", ans);
  for (int i = 0; i < ans; i++) {
    printf("%d %d ", destroy[i].first, destroy[i].second);
    printf("%d %d\n", comp[i][0], comp[i + 1][0]);
  }
  return 0;
}
