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 N = 2e5 + 5, inf = 1e18;
long long n, tmp1, tmp2, par[N], V, c;
bool mark[N], tmp;
vector<long long> vec, adj[N];
pair<long long, long long> P[N];
long long get_par(long long v) {
  return (par[v] == -1 ? v : (par[v] = get_par(par[v])));
}
bool marge(long long v, long long u) {
  v = get_par(v);
  u = get_par(u);
  if (u == v) return true;
  par[u] = v;
  return false;
}
void DFS(long long v, long long p) {
  mark[v] = true;
  for (auto u : adj[v]) {
    if (u == p) continue;
    if (mark[u] != true)
      DFS(u, v);
    else
      tmp = false;
  }
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  fill(par, par + n + 1, -1);
  for (long long i = 1; i < n; i++) {
    cin >> tmp1 >> tmp2;
    adj[tmp1].push_back(tmp2);
    adj[tmp2].push_back(tmp1);
    if (marge(tmp1, tmp2)) {
      P[c] = {tmp1, tmp2};
      c++;
    }
  }
  for (long long i = 1; i <= n; i++) {
    if (mark[i] != true) {
      vec.push_back(i);
      tmp = true;
      DFS(i, 0);
      if (tmp) V = i;
    }
  }
  cout << vec.size() - 1 << endl;
  for (long long i = 0; i < vec.size(); i++) {
    long long X = vec[i];
    if (X == V) continue;
    cout << P[i].first << " " << P[i].second << " " << V << " " << X << endl;
  }
  return 0;
}
