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 long long int INF = 2000000000000000000;
vector<vector<int>> adj;
vector<vector<int>> comp;
vector<int> visited;
vector<pair<int, int>> cycledge;
vector<int> parent;
vector<int> disc;
int t = 0;
int m = 0;
long long int MOD(long long int a, long long int b) {
  if (a > b)
    return a - b;
  else
    return b - a;
}
long long int max3(long long int a, long long int b, long long int c) {
  return max(c, max(a, b));
}
long long int min3(long long int a, long long int b, long long int c) {
  return min(a, min(b, c));
}
long long int power(long long int x, long long int y) {
  long long int res = 1;
  x = x;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
long long int logg(long long int a) {
  long long int x = 0;
  while (a > 1) {
    x++;
    a /= 2;
  }
  return x;
}
long long int gcd(long long int a, long long int b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
void dfs2(int i) {
  visited[i] = 1;
  comp[m].push_back(i);
  disc[i] = t;
  t++;
  for (int j = 0; j < adj[i].size(); j++) {
    if (visited[adj[i][j]] == 0) {
      parent[adj[i][j]] = i;
      dfs2(adj[i][j]);
    } else {
      if (parent[i] != adj[i][j] && disc[i] > disc[adj[i][j]]) {
        cycledge.push_back(make_pair(i, adj[i][j]));
      }
    }
  }
}
void dfs(int n) {
  for (int i = 1; i <= n; i++) {
    if (visited[i] == 0) {
      t = 1;
      dfs2(i);
      m++;
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  int n;
  cin >> n;
  adj.resize(n + 1);
  comp.resize(n);
  visited.resize(n + 1, 0);
  parent.resize(n + 1, 0);
  disc.resize(n + 1, 0);
  for (int i = 0; i < n - 1; i++) {
    int x, y;
    cin >> x >> y;
    adj[x].push_back(y);
    adj[y].push_back(x);
  }
  dfs(n);
  cout << cycledge.size() << endl;
  for (int i = 0; i < cycledge.size(); i++) {
    cout << cycledge[i].first << ' ' << cycledge[i].second << ' '
         << *comp[i].begin() << ' ' << *comp[i + 1].begin();
  }
}
