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;
ifstream inf("abc.txt");
ofstream outf("op.txt");
int a[1005][1005];
void dfs(int s, vector<vector<int> > &g, vector<int> &visited, int mark) {
  visited[s] = mark;
  for (int i = 0; i < g[s].size(); i++) {
    if (!visited[g[s][i]]) {
      a[s][g[s][i]] = 1;
      a[g[s][i]][s] = 1;
      dfs(g[s][i], g, visited, mark);
    }
  }
}
int check[1005][1005];
int main() {
  int n, x, y;
  cin >> n;
  vector<vector<int> > g(n + 1);
  for (int i = 1; i < n; i++) {
    cin >> x >> y;
    g[x].push_back(y);
    g[y].push_back(x);
    a[x][y] = -1;
    a[y][x] = -1;
  }
  vector<int> visited(n + 1);
  int mark = 1;
  vector<int> cr;
  cr.push_back(-1);
  for (int i = 1; i <= n; i++) {
    if (!visited[i]) {
      cr.push_back(i);
      dfs(i, g, visited, mark++);
    }
  }
  vector<pair<int, int> > r;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      if (a[i][j] == -1) {
        r.push_back(make_pair(i, j));
      }
    }
  }
  cout << r.size() << endl;
  vector<vector<pair<int, int> > > c(mark);
  for (int i = 0; i < r.size(); i++) {
    c[visited[r[i].first]].push_back(make_pair(r[i].first, r[i].second));
  }
  for (int i = 1; i < mark; i++) {
    for (int j = i + 1; j < mark; j++) {
      if (check[i][j] == 0) {
        bool flag = false;
        for (int k = 0; k < c[i].size(); k++) {
          int v1 = c[i][k].first;
          int v2 = c[i][k].second;
          if (a[v1][v2] == -1) {
            cout << v1 << " " << v2 << " " << v1 << " " << cr[j] << endl;
            flag = true;
            check[i][j] = 1;
            a[v1][v2] = 1;
            break;
          }
        }
        if (flag == false) {
          for (int k = 0; k < c[j].size(); k++) {
            int v1 = c[j][k].first;
            int v2 = c[j][k].second;
            if (a[v1][v2] == -1) {
              cout << v1 << " " << v2 << " " << v1 << " " << cr[i] << endl;
              flag = true;
              check[i][j] = 1;
              a[v1][v2] = 1;
              break;
            }
          }
        }
      }
    }
  }
  return 0;
}
