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;
vector<vector<int> > G;
int main() {
  int n = ({
    int t;
    scanf("%d", &t);
    t;
  });
  G.resize(n);
  int u, v;
  for (int i = 0; i < n - 1; i++) {
    u = ({
          int t;
          scanf("%d", &t);
          t;
        }) -
        1,
    v = ({
          int t;
          scanf("%d", &t);
          t;
        }) -
        1;
    G[u].push_back(v);
    G[v].push_back(u);
  }
  vector<pair<int, int> > rem, nrd;
  int seen[n], p[n];
  int prev = -1;
  for (int i = 0; i < n; i++) seen[i] = 0, p[i] = -1;
  for (int i = 0; i < n; i++) {
    if (!seen[i]) {
      seen[i] = 1;
      if (prev != -1) {
        nrd.push_back(pair<int, int>(prev, i));
      }
      prev = i;
      queue<int> q;
      q.push(i);
      seen[i] = 1;
      while (!q.empty()) {
        int cur = q.front();
        q.pop();
        for (int j = 0; j < (int)(G[cur].size()); j++) {
          if (seen[G[cur][j]]) {
            if (G[cur][j] != p[cur])
              rem.push_back(pair<int, int>(cur, G[cur][j]));
          } else {
            seen[G[cur][j]] = 1;
            p[G[cur][j]] = cur;
            q.push(G[cur][j]);
          }
        }
      }
    }
  }
  int m = (int)(nrd.size());
  set<pair<int, int> > ss;
  for (int i = 0; i < (int)(rem.size()); i++)
    ss.insert(pair<int, int>(min(rem[i].first, rem[i].second),
                             max(rem[i].first, rem[i].second)));
  assert(m == (int)(ss.size()));
  vector<pair<int, int> > remi;
  for (__typeof__(ss.begin()) it = ss.begin(); it != ss.end(); it++)
    remi.push_back(*it);
  cout << m << endl;
  for (int i = 0; i < m; i++)
    cout << remi[i].first + 1 << " " << remi[i].second + 1 << " "
         << nrd[i].first + 1 << " " << nrd[i].second + 1 << endl;
  return 0;
}
