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>
#pragma comment(linker, "/STACK:25600000")
using namespace std;
const int undef = -1;
int n = 0;
int visited[1000];
vector<list<int> > graph;
vector<int> delegates;
set<pair<int, int> > road_list;
void init() {
  delegates.reserve(1000 + 10);
  graph.reserve(1000 + 10);
  for (int i = 0; i < 1000; ++i) {
    visited[i] = undef;
    graph.push_back(list<int>());
    delegates.push_back(undef);
  }
  cin >> n;
  int a = 0;
  int b = 0;
  for (int i = 1; i < n; ++i) {
    cin >> a >> b;
    graph[a - 1].push_back(b - 1);
    graph[b - 1].push_back(a - 1);
  }
}
void explore(int, int, int);
void solve() {
  int comp = 0;
  for (int i = 0; i < n; ++i) {
    if (visited[i] == undef) {
      explore(i, comp, undef);
      delegates[comp] = i;
      comp++;
    }
  }
  cout << comp - 1 << endl;
  int i = 0;
  while (comp - 1 != 0) {
    set<pair<int, int> >::iterator sIt = road_list.begin();
    pair<int, int> p = *sIt;
    cout << p.first + 1 << " " << p.second + 1 << " ";
    cout << delegates[i] + 1 << " " << delegates[i + 1] + 1 << endl;
    road_list.erase(sIt);
    ++i;
    --comp;
  }
}
void explore(int i, int component, int comefrom) {
  visited[i] = component;
  for (list<int>::iterator it = graph[i].begin(); it != graph[i].end(); ++it) {
    if (visited[*it] == undef) {
      explore(*it, component, i);
    } else if (comefrom != *it) {
      if (road_list.find(make_pair(*it, i)) == road_list.end()) {
        road_list.insert(make_pair(i, *it));
      }
    }
  }
}
int main() {
  init();
  solve();
  return 0;
}
