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;
struct UFDS {
  int n;
  vector<int> sz, par;
  set<int> s;
  void init(int x) {
    n = x;
    par.resize(n + 1);
    sz.resize(n + 1, 1);
    iota(par.begin(), par.end(), 0);
  }
  int find(int v) { return par[v] == v ? v : par[v] = find(par[v]); }
  bool join(int a, int b) {
    a = find(a);
    b = find(b);
    if (a == b) return false;
    if (sz[a] < sz[b]) swap(a, b);
    par[b] = a;
    sz[a] += sz[b];
    return true;
  }
  bool check() {
    for (int i = 1; i <= n; i++) s.insert(find(i));
    if (s.size() == 1)
      return true;
    else
      return false;
  }
  set<int> unique() { return s; }
};
int main() {
  int n;
  cin >> n;
  UFDS u;
  u.init(n);
  vector<pair<int, int>> v;
  int a, b;
  for (int i = 1; i <= n; i++) {
    cin >> a >> b;
    if (!u.join(a, b)) {
      v.push_back({a, b});
    }
  }
  if (u.check())
    cout << 0;
  else {
    set<int> s = u.unique();
    int ans = 0;
    vector<pair<int, int>> l, m;
    for (int i = 0; i <= v.size() - 1; i++) {
      if (*s.begin() != 0) {
        ans++;
        l.push_back(make_pair(v[i].first, v[i].second));
        int p = *s.begin();
        s.erase(*s.begin());
        int q = *s.begin();
        s.erase(*s.begin());
        m.push_back(make_pair(p, q));
      }
    }
    cout << ans << "\n";
    for (int i = 0; i <= (int)l.size() - 1; i++) {
      cout << l[i].first << " " << l[i].second << " " << m[i].first << " "
           << m[i].second << "\n";
    }
  }
  return 0;
}
