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;
list<int> adjlist[1005];
bool visited[1005];
int root[1005];
int cmp;
vector<pair<int, int> > v[1005];
int ti = 0;
int arr[1005];
int leader[1005];
int sz[1005];
void init(int n) {
  for (int i = 1; i <= n; i++) {
    leader[i] = i;
    sz[i] = 1;
  }
}
int getleader(int x) {
  int y = x;
  while (leader[y] != y) {
    y = leader[y];
  }
  while (leader[x] != x) {
    int t = x;
    x = leader[x];
    leader[t] = y;
  }
  return y;
}
void uni(int x, int y) {
  if (sz[x] > sz[y]) {
    leader[y] = x;
    sz[x] += sz[y];
  } else {
    leader[x] = y;
    sz[y] += sz[x];
  }
}
void dfs(int s, int p) {
  visited[s] = 1;
  arr[s] = ti++;
  list<int>::iterator it;
  for (it = adjlist[s].begin(); it != adjlist[s].end(); it++) {
    if (visited[*it]) {
      if (*it != p && arr[s] > arr[*it]) {
        v[cmp].push_back(make_pair(s, *it));
      }
    } else
      dfs(*it, s);
  }
  ti++;
}
int main() {
  int n, i, j, k;
  scanf("%d", &n);
  for (i = 0; i < n - 1; i++) {
    scanf("%d %d", &k, &j);
    adjlist[j].push_back(k);
    adjlist[k].push_back(j);
  }
  for (i = 1; i <= n; i++) {
    if (!visited[i]) {
      cmp++;
      root[cmp] = i;
      dfs(i, 0);
    }
  }
  init(cmp);
  printf("%d\n", cmp - 1);
  for (i = 1; i <= cmp; i++) {
    for (j = 0; j < (int)v[i].size(); j++) {
      for (k = 1; k <= cmp; k++) {
        if (getleader(k) != getleader(i)) break;
      }
      printf("%d %d %d %d\n", v[i][j].first, v[i][j].second, v[i][j].first,
             root[k]);
      uni(getleader(k), getleader(i));
    }
  }
  return 0;
}
