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 dx[] = {0, 0, 1, -1};
int dy[] = {1, -1, 0, 0};
int dx8[] = {0, 0, 1, 1, 1, -1, -1, -1};
int dy8[] = {1, -1, -1, 0, 1, -1, 0, 1};
int kx8[] = {1, 1, 2, 2, -1, -1, -2, -2};
int ky8[] = {2, -2, 1, -1, 2, -2, 1, -1};
long long bigmod(long long a, long long b, long long c) {
  if (b == 0) return 1 % c;
  long long x = bigmod(a, b / 2, c);
  x = (x * x) % c;
  if (b % 2 == 1) x = (x * a) % c;
  return x;
}
long long mod_inverse(long long a, long long mod) {
  return bigmod(a, mod - 2, mod);
}
int par[1005];
vector<pair<int, int> > vc;
long long unfn(int a) {
  if (a == par[a]) return a;
  return par[a] = unfn(par[a]);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) par[i] = i;
  for (int i = 0; i < (n - 1); i++) {
    int x, y;
    cin >> x >> y;
    int u = unfn(x);
    int v = unfn(y);
    if (u == v) {
      vc.push_back({x, y});
      continue;
    }
    par[u] = v;
  }
  int pre = 1;
  cout << vc.size() << endl;
  int id = 0;
  for (int i = 1; i <= n; i++) {
    int u = unfn(i);
    int v = unfn(pre);
    if (u != v) {
      cout << vc[id].first << " " << vc[id].second << " " << u << " " << v
           << endl;
      id++;
      par[v] = u;
    }
    pre = u;
  }
  return 0;
}
