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;
void dfs(int i, vector<vector<pair<int, bool> > >& edg, vector<bool>& vis,
         vector<int>& par) {
  vis[i] = true;
  for (int j = 0; j < edg[i].size(); j++) {
    if (!vis[edg[i][j].first]) {
      par[edg[i][j].first] = par[i];
      edg[i][j].second = true;
      for (int k = 0; k < edg[edg[i][j].first].size(); k++) {
        if (edg[edg[i][j].first][k].first == i) {
          edg[edg[i][j].first][k].second = true;
          break;
        }
      }
      dfs(edg[i][j].first, edg, vis, par);
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  ;
  int n;
  cin >> n;
  vector<vector<pair<int, bool> > > edg(n + 1);
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    edg[u].push_back(make_pair(v, false));
    edg[v].push_back(make_pair(u, false));
  }
  vector<bool> vis(n + 1, false);
  vector<int> par(n + 1, -1);
  for (int i = 1; i < n + 1; i++) {
    if (!vis[i]) {
      par[i] = i;
      dfs(i, edg, vis, par);
    }
  }
  int count = 0;
  for (int i = 1; i < n + 1; i++) {
    if (par[i] == 1) {
      count++;
    }
  }
  if (count == n) {
    cout << 0 << endl;
  } else {
    set<int> x;
    for (int i = 1; i < n + 1; i++) {
      if (par[i] != 1) {
        x.insert(par[i]);
      }
    }
    vector<pair<int, int> > unusedEdges;
    for (int i = 1; i < n + 1; i++) {
      for (int j = 0; j < edg[i].size(); j++) {
        if (!edg[i][j].second) {
          unusedEdges.push_back(make_pair(i, edg[i][j].first));
          edg[i][j].second = true;
          for (int k = 0; k < edg[edg[i][j].first].size(); k++) {
            if (edg[edg[i][j].first][k].first == i) {
              edg[edg[i][j].first][k].second = true;
              break;
            }
          }
        }
      }
    }
    set<int>::iterator it;
    int z = 0;
    cout << x.size() << endl;
    for (it = x.begin(); it != x.end(); it++) {
      cout << unusedEdges[z].first << " " << unusedEdges[z].second << " ";
      cout << 1 << " " << *it << endl;
      z++;
    }
    cout << endl;
  }
}
