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<int> p, r, c;
vector<pair<int, int>> rd;
int parent(int x) {
  if (x == p[x]) return x;
  return p[x] = parent(p[x]);
}
bool same_set(int xx, int yy) {
  int x = parent(xx), y = parent(yy);
  if (x != y) {
    if (r[x] > r[y]) swap(x, y);
    p[x] = y;
    if (r[x] == r[y]) r[y]++;
  }
  return x != y;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  int n;
  cin >> n;
  p.resize(n + 1);
  r.resize(n + 1);
  for (long long i = 1; i <= (long long)n; ++i) {
    r[i] = 0;
    p[i] = i;
  }
  for (long long i = 1; i <= (long long)n - 1; ++i) {
    int x, y;
    cin >> x >> y;
    bool f = same_set(x, y);
    if (!f) rd.push_back({x, y});
  }
  sort(p.begin() + 1, p.begin() + n + 1);
  for (long long i = 1; i <= (long long)n; ++i) {
    int j = i;
    c.push_back(p[i]);
    while (j <= n && p[i] == p[j]) j++;
    i = j - 1;
  }
  cout << rd.size() << endl;
  for (long long i = 0; i <= (long long)rd.size() - 1; ++i)
    cout << rd[i].first << " " << rd[i].second << " " << c[i] << " " << c[i + 1]
         << endl;
}
