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;
const int mxN = 1000;
int prnt[mxN];
int rnk[mxN];
vector<int> adj[mxN];
vector<pair<int, int>> ans;
int vis[mxN];
int find(int i) {
  if (prnt[i] != i) prnt[i] = find(prnt[i]);
  return prnt[i];
}
void join(int x, int y) {
  int xroot = find(x);
  int yroot = find(y);
  if (rnk[xroot] < rnk[yroot])
    prnt[xroot] = yroot;
  else if (rnk[xroot] > rnk[yroot])
    prnt[yroot] = xroot;
  else {
    prnt[yroot] = xroot;
    rnk[xroot]++;
  }
}
void dfs(int u, int e) {
  for (auto ad = adj[u].begin(); ad != adj[u].end(); ad++) {
    if (*ad == e || vis[*ad]) continue;
    int adprnt = find(*ad);
    int uprnt = find(u);
    if (adprnt == uprnt) {
      ans.push_back(make_pair(u, *ad));
      continue;
    }
    join(*ad, u);
    dfs(*ad, u);
  }
  vis[u] = 1;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    adj[x].push_back(y);
    adj[y].push_back(x);
  }
  for (int i = 0; i < n; i++) {
    prnt[i] = i;
  }
  for (int i = 0; i < n; i++) {
    if (!vis[i]) {
      dfs(i, -1);
    }
  }
  cout << ans.size() << endl;
  int id = 0;
  for (int i = 0; i < n; i++) {
    for (int j = i + 1; j < n; j++) {
      int x = find(i);
      int y = find(j);
      if (x != y) {
        cout << ans[id].first + 1 << " " << ans[id].second + 1 << " " << i + 1
             << " " << j + 1 << endl;
        id++;
        join(i, j);
      }
    }
  }
  return 0;
}
