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 par[1234];
int sz[1234];
int a[1234][2];
bool vis[1234];
vector<int> edj, cmp;
stack<int> stk;
void init(int n) {
  for (int i = 1; i <= n; i++) par[i] = i;
}
int find(int x) {
  if (x == par[x]) return x;
  return par[x] = find(par[x]);
}
bool comp(int x, int y) {
  x = find(a[x][0]);
  y = find(a[y][0]);
  return (x > y);
}
bool join(int x, int y) {
  x = find(x);
  y = find(y);
  if (x == y) return true;
  if (sz[x] > sz[y]) {
    par[y] = x;
    sz[x] += sz[y] + 1;
  } else {
    par[x] = y;
    sz[y] += sz[x] + 1;
  }
  return false;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  init(n);
  int x, y;
  for (int i = 1; i < n; i++) {
    cin >> x >> y;
    a[i][0] = x;
    a[i][1] = y;
    if (join(x, y)) edj.push_back(i);
  }
  for (int i = 1; i <= n; i++) {
    x = find(i);
    if (vis[x]) continue;
    vis[x] = true;
    cmp.push_back(x);
  }
  int z = edj.size();
  for (int i = 0; i < z; i++) {
    x = edj[i];
    y = a[x][0];
    x = find(y);
    vis[x] = false;
  }
  int lst = -1, k;
  z = cmp.size();
  for (int i = 0; i < z; i++) {
    x = cmp[i];
    if (vis[x]) stk.push(x);
  }
  z = edj.size();
  sort(edj.begin(), edj.end(), comp);
  cout << z << endl;
  for (int i = 0; i < z; i++) {
    x = a[edj[i]][0];
    y = find(x);
    k = stk.top();
    stk.pop();
    if (y != lst && lst != -1) stk.push(y);
    lst = y;
    cout << a[x][0] << " " << a[x][1] << " " << a[x][0] << " " << k << endl;
  }
  return 0;
}
