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>
struct day {
  int a;
  int b;
  int a1;
  int b1;
};
int counter = 0;
using namespace std;
int n;
vector<vector<int> > g;
vector<bool> used;
vector<int> g1;
vector<day> t;
void dfs(int v) {
  used[v] = 1;
  for (auto i : g[v]) {
    if (!used[i]) dfs(i);
  }
}
map<pair<int, int>, bool> used1;
void dfs1(int v, int p) {
  cout << v << endl;
  used[v] = 1;
  for (auto i : g[v]) {
    if (i != p) {
      if (used[i] == 1 && used1[{max(i, v), min(i, v)}] == 0) {
        used1[{max(i, v), min(i, v)}] = 1;
        cout << i << " " << v << "<----" << endl;
        t.push_back({i, v, g1[counter], g1[counter + 1]});
        counter++;
      } else {
        dfs1(i, v);
      }
    }
  }
}
int main() {
  cin >> n;
  g.resize(n + 1);
  used.resize(n + 1, 0);
  {
    int a, b;
    for (int i = 0; i < n - 1; i++) {
      cin >> a >> b;
      g[a].push_back(b);
      g[b].push_back(a);
    }
  }
  for (int i = 1; i <= n; i++) {
    if (used[i] == 0) {
      g1.push_back(i);
      dfs(i);
    }
  }
  used.clear();
  used.resize(n + 1, 0);
  for (int i = 1; i <= n; i++) {
    if (used[i] == 0) {
      dfs1(i, 0);
    }
  }
  cout << t.size() << endl;
  for (int i = 0; i < t.size(); i++) {
    cout << t[i].a << " " << t[i].b << " " << t[i].a1 << " " << t[i].b1 << endl;
  }
}
