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;
int n, mark;
vector<set<int> > s;
int visited[1009];
struct node {
  int ii, jj;
  int xx, yy;
};
vector<struct node> ss;
int ii, jj;
void print() {
  cout << ss.size() << endl;
  int i;
  for (i = (0); i < (ss.size()); ++i) {
    cout << ss[i].ii + 1 << " " << ss[i].jj + 1 << " " << ss[i].xx + 1 << " "
         << ss[i].yy + 1 << endl;
  }
}
bool check(int n) {
  int i;
  for (i = (0); i < (n); ++i) {
    if (visited[i] == 0) {
      return false;
    }
  }
  return true;
}
void dfs(int index) {
  if (visited[index] == 1) {
    return;
  }
  visited[index] = 1;
  int i;
  set<int>::iterator it;
  for (it = s[index].begin(); it != s[index].end(); it++) {
    dfs((*it));
  }
}
void f(int index, int parent) {
  if (visited[index] != 0) {
    return;
  }
  visited[index] = mark;
  set<int>::iterator it;
  for (it = s[index].begin(); it != s[index].end(); it++) {
    int x = (*it);
    if (x == parent) {
      continue;
    }
    if (visited[x] != 0) {
      if (ii == -1) {
        ii = index;
        jj = x;
      }
    }
    f(x, index);
  }
}
int get(int n) {
  int i;
  for (i = (0); i < (n); ++i) {
    if (visited[i] == 0) {
      return i;
    }
  }
}
void update(int a, int b, int c, int d) {
  s[a].erase(b);
  s[b].erase(a);
  s[c].insert(d);
  s[d].insert(c);
}
int find(int index, int n) {
  int i;
  for (i = (0); i < (n); ++i) {
    if (visited[i] != visited[index]) {
      return i;
    }
  }
}
int main() {
  scanf("%d", &n);
  s.resize(n);
  int i;
  for (i = (0); i < (n - 1); ++i) {
    int x, y;
    scanf("%d", &x);
    scanf("%d", &y);
    x--;
    y--;
    s[x].insert(y);
    s[y].insert(x);
  }
  while (1) {
    memset(visited, 0, sizeof(visited));
    dfs(0);
    if (check(n)) {
      break;
    }
    ii = -1;
    jj = -1;
    memset(visited, 0, sizeof(visited));
    int i;
    mark = 1;
    for (i = (0); i < (n); ++i) {
      if (visited[i] != 0) {
        continue;
      }
      f(i, i);
      mark++;
    }
    struct node ab;
    ab.ii = ii;
    ab.jj = jj;
    ab.xx = ii;
    ab.yy = find(ii, n);
    ss.push_back(ab);
    update(ab.ii, ab.jj, ab.xx, ab.yy);
  }
  print();
  return 0;
}
