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;
vector<int> ady[1005];
int dfsNum[1005], V, low[1005], num, seen[1005], num_component,
    visto[1005][1005], root[1005];
vector<pair<int, int> > back_edge[1005];
void dfs(int x, int parent) {
  dfsNum[x] = low[x] = num++;
  seen[x] = 1;
  for (int i = 0; i < ady[x].size(); ++i) {
    int y = ady[x][i];
    if (!seen[y]) {
      dfs(y, x);
      if (low[x] > low[y]) low[x] = low[y];
    } else if (y != parent) {
      if (!visto[x][y] && !visto[y][x]) {
        visto[x][y] = visto[y][x] = 1;
        back_edge[num_component].push_back(make_pair(x, y));
      }
      low[x] = (low[x] > dfsNum[y]) ? dfsNum[y] : low[x];
    }
  }
}
int uf[1005];
void MakeSet() {
  for (int i = 1; i <= V; ++i) uf[i] = i;
}
int Find(int x) { return (uf[x] == x) ? x : uf[x] = Find(uf[x]); }
void Union(int x, int y) { uf[Find(x)] = Find(y); }
int main() {
  int t, u, v, i, j;
  scanf("%d", &V);
  MakeSet();
  for (i = 0; i < V - 1; ++i) {
    scanf("%d %d", &u, &v);
    ady[u].push_back(v);
    ady[v].push_back(u);
    Union(u, v);
  }
  memset(seen, 0, sizeof(seen));
  memset(visto, 0, sizeof(visto));
  num_component = 0;
  for (i = 1; i <= V; ++i) {
    if (!seen[i]) {
      num = 1;
      dfs(i, -1);
      root[num_component++] = i;
    }
  }
  printf("%d\n", num_component - 1);
  for (i = 0; i < num_component; ++i) {
    for (j = 0; j < num_component; ++j) {
      if (i == j) continue;
      if (Find(root[i]) != Find(root[j])) {
        if (back_edge[i].size() > 0) {
          pair<int, int> pii = back_edge[i].back();
          back_edge[i].pop_back();
          printf("%d %d %d %d\n", pii.first, pii.second, pii.first, root[j]);
          Union(root[i], root[j]);
        } else if (back_edge[j].size() > 0) {
          pair<int, int> pii = back_edge[j].back();
          back_edge[j].pop_back();
          printf("%d %d %d %d\n", pii.first, pii.second, pii.first, root[i]);
          Union(root[i], root[j]);
        }
      }
    }
  }
  return 0;
}
