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;
const int inf = 1999999999;
const double pi = acos(-1.0);
const double eps = 1e-10;
char gc() {
  char c;
  while (isspace(c = getchar()))
    ;
  return c;
}
int gs(char* s) {
  gets(s);
  int l = strlen(s);
  while (l && isspace(s[l - 1])) s[--l] = 0;
  return l;
}
vector<int> adj[1000];
vector<pair<int, int> > ins, rem;
bool vis[1000], pres[1000];
int s, e;
void dfs(int a, int p) {
  pres[a] = true;
  for (int i = 0; i < ((int)(adj[a]).size()); i++) {
    int b = adj[a][i];
    if (!vis[b]) {
      vis[b] = true;
      dfs(b, a);
    } else if (p != b && pres[b]) {
      s = a;
      e = b;
    }
  }
  pres[a] = false;
}
void flood(int a) {
  for (int i = 0; i < ((int)(adj[a]).size()); i++) {
    int b = adj[a][i];
    if (!vis[b]) {
      vis[b] = true;
      flood(b);
    }
  }
}
int main() {
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    adj[a].push_back(b);
    adj[b].push_back(a);
  }
  while (true) {
    memset((vis), (false), sizeof(vis));
    memset((pres), (false), sizeof(pres));
    s = -1;
    for (int i = 0; i < n; i++) {
      if (!vis[i]) {
        vis[i] = true;
        dfs(i, -1);
        if (s != -1) break;
      }
    }
    if (s == -1) break;
    adj[s].erase(find((adj[s]).begin(), (adj[s]).end(), e));
    adj[e].erase(find((adj[e]).begin(), (adj[e]).end(), s));
    rem.push_back(make_pair(s, e));
  }
  {
    memset((vis), (false), sizeof(vis));
    vector<int> a;
    for (int i = 0; i < n; i++) {
      if (!vis[i]) {
        vis[i] = true;
        a.push_back(i);
        flood(i);
      }
    }
    for (int i = 0; i < ((int)(a).size()) - 1; i++)
      ins.push_back(make_pair(a[i], a[i + 1]));
  }
  cout << ((int)(ins).size()) << ('\n');
  for (int i = 0; i < ((int)(ins).size()); i++)
    cout << rem[i].first + 1 << ' ' << rem[i].second + 1 << ' '
         << ins[i].first + 1 << ' ' << ins[i].second + 1 << ' ';
  return 0;
}
