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, a, b, compLeader[1002], compAmount, vis[1002], myComp[1002];
vector<int> graph[1002];
void visit(int node, int leader) {
  myComp[node] = compAmount;
  for (int son : graph[node]) {
    if (!myComp[son]) visit(son, leader);
  }
}
void CountLeaders() {
  int t = -1;
  for (int i = 1; i <= n; i++) {
    if (!myComp[i]) {
      compLeader[++compAmount] = i, visit(i, i), t++;
    }
  }
  cout << t << "\n";
}
bool StillConnected(int node, int a, int b, int color) {
  vis[node] = color;
  if (node == b) return true;
  bool ans = false;
  for (int son : graph[node]) {
    if (son && (node != a || son != b) && vis[son] != color &&
        (node != b || son != a)) {
      ans |= StillConnected(son, a, b, color);
    }
  }
  return ans;
}
void Disconnect(int a, int b) {
  for (int i = 0; i < graph[a].size(); i++)
    if (graph[a][i] == b) graph[a][i] = 0;
}
int parent[1002], sz[1002];
void initialize() {
  for (int i = 1; i <= n; i++) parent[i] = i, sz[i] = 1;
}
int find(int x) {
  while (parent[x] != x) x = parent[x] = parent[parent[x]];
  return x;
}
bool joined(int a, int b) { return find(a) == find(b); }
void join(int a, int b) {
  int aRoot = find(a), bRoot = find(b);
  if (sz[aRoot] > sz[bRoot]) swap(aRoot, bRoot);
  sz[bRoot] += sz[aRoot];
  parent[aRoot] = bRoot;
}
void PrintNewRoads() {
  int color = 1;
  for (int i = 1; i <= n; i++) {
    for (int son : graph[i]) {
      int chosenComp = 1;
      while (joined(chosenComp, myComp[i])) chosenComp++;
      if (chosenComp <= compAmount && StillConnected(i, i, son, color++)) {
        join(chosenComp, myComp[i]);
        cout << i << " " << son << " " << i << " " << compLeader[chosenComp]
             << "\n";
        Disconnect(i, son), Disconnect(son, i);
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  initialize();
  for (int i = 1; i < n; i++)
    cin >> a >> b, graph[a].push_back(b), graph[b].push_back(a);
  CountLeaders();
  PrintNewRoads();
  return 0;
}
