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 MAXN = 1010;
int pai[MAXN], peso[MAXN], qtd[MAXN], qtd_edge[MAXN], grau[MAXN];
bool mark[MAXN], aresta[MAXN][MAXN];
vector<pair<int, int> > tira, coloca;
int find(int x) {
  if (pai[x] == x) return x;
  return pai[x] = find(pai[x]);
}
void join(int x, int y) {
  x = find(x), y = find(y);
  if (x == y) return;
  if (peso[x] > peso[y]) {
    pai[y] = x;
    qtd[x] += qtd[y];
  } else if (peso[y] > peso[x]) {
    pai[x] = y;
    qtd[y] += qtd[x];
  } else {
    pai[x] = y;
    peso[y]++;
    qtd[y] += qtd[x];
  }
}
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    pai[i] = i;
    qtd[i] = 1;
  }
  for (int i = 1; i <= n - 1; i++) {
    int u, v;
    cin >> u >> v;
    join(u, v);
    aresta[u][v] = aresta[v][u] = 1;
    grau[u]++, grau[v]++;
  }
  for (int i = 1; i <= n; i++) qtd_edge[find(i)] += grau[i];
  for (int i = 1; i <= n; i++) {
    if (!mark[find(i)]) {
      qtd_edge[find(i)] /= 2;
      mark[find(i)] = 1;
    }
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      if (aresta[i][j] && grau[i] > 1 && grau[j] > 1 &&
          qtd_edge[find(i)] > qtd[find(i)] - 1) {
        aresta[i][j] = aresta[j][i] = 0;
        grau[i]--, grau[j]--;
        tira.push_back(make_pair(i, j));
        qtd_edge[find(i)]--;
        ans++;
      }
    }
  }
  int z = 0;
  memset(mark, 0, sizeof mark);
  for (int i = 1; i <= n; i++) {
    if (mark[find(i)])
      continue;
    else
      mark[find(i)] = 1;
    for (int j = i + 1; j <= n; j++) {
      if (find(i) != find(j) && z < ans) {
        join(i, j);
        z++;
        coloca.push_back(make_pair(i, j));
        if (z == ans) break;
      }
    }
  }
  cout << ans << "\n";
  for (int i = 0; i < tira.size(); i++)
    cout << tira[i].first << " " << tira[i].second << " " << coloca[i].first
         << " " << coloca[i].second << "\n";
}
