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 n;
int A[1005];
int m;
int visited[1005];
vector<int> adj[1005];
vector<pair<int, int>> unused;
vector<int> plans;
map<pair<int, int>, int> used;
void initialize(int Arr[], int N) {
  for (int i = 0; i < N; i++) Arr[i] = i;
}
bool Find(int Arr[], int A, int B) {
  if (Arr[A] == Arr[B])
    return true;
  else
    return false;
}
void Union(int Arr[], int N, int A, int B) {
  int TEMP = Arr[A];
  for (int i = 0; i < N; i++) {
    if (Arr[i] == TEMP) Arr[i] = Arr[B];
  }
}
void dfs(int index, int parent) {
  visited[index] = 1;
  for (int i = 0; i < adj[index].size(); i++) {
    if (visited[adj[index][i]]) {
      if (adj[index][i] != parent)
        unused.push_back(make_pair(index, adj[index][i]));
    } else
      dfs(adj[index][i], index);
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  n++;
  initialize(A, n);
  for (int i = 0; i < n - 2; i++) {
    int x, y;
    cin >> x >> y;
    adj[x].push_back(y);
    adj[y].push_back(x);
    Union(A, n, x, y);
  }
  for (int i = 1; i < n; i++)
    if (!visited[i]) dfs(i, i);
  int counter = -1;
  for (int i = 1; i < n; i++)
    if (A[i] == i) counter++;
  cout << counter << endl;
  int flag = 0, root;
  for (int i = 1; i < n; i++)
    if (A[i] == i && !flag++)
      root = i;
    else if (A[i] == i)
      plans.push_back(i);
  while (counter--) {
    pair<int, int> temp;
    flag = 0;
    while (!flag) {
      pair<int, int> temp2;
      temp = unused.back();
      temp2.second = temp.first;
      temp2.first = temp.second;
      if (!used[temp] && !used[temp2]) flag = 1;
      used[temp] = 1;
      used[temp2] = 1;
      unused.pop_back();
    }
    int to = plans.back();
    plans.pop_back();
    cout << temp.first << " " << temp.second << " " << root << " " << to
         << endl;
  }
  return 0;
}
