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 dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};
const int N = 2e3 + 5, M = 27;
int par[N], sz[N], n, m;
set<int> roots;
vector<pair<int, int>> edges;
int find(int x) {
  if (par[x] == x) return x;
  return par[x] = find(par[x]);
}
bool connect(int a, int b) {
  int u = find(a), v = find(b);
  if (u == v) {
    edges.push_back({a, b});
    return 0;
  }
  if (sz[u] > sz[v]) {
    par[v] = u;
    roots.erase(v);
    sz[u] += sz[v];
  } else {
    par[u] = v;
    roots.erase(u);
    sz[v] += sz[u];
  }
  return 1;
}
void init() {
  roots.clear();
  for (int(i) = 1; i <= (n); i++) {
    roots.insert(i);
    par[i] = i;
    sz[i] = 1;
  }
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  cin >> n;
  init();
  int a, b;
  for (int(i) = 0; i < (n - 1); i++) {
    cin >> a >> b;
    connect(a, b);
  }
  cout << edges.size() << "\n";
  vector<int> vv((roots).begin(), (roots).end());
  for (int(i) = 0; i < (vv.size() - 1); i++) {
    cout << edges[i].first << " " << edges[i].second << " " << vv[i] << " "
         << vv[i + 1] << "\n";
  }
}
