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 GCC optimize("-Ofast")
#pragma GCC target( \
    "sse,sse2,sse3,ssse3,sse4,sse4.2,popcnt,abm,mmx,avx2,tune=native")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-funroll-all-loops,-fpeel-loops,-funswitch-loops")
using namespace std;
short dx[] = {1, -1, 0, 0};
short dy[] = {0, 0, 1, -1};
struct edge {
  int to;
  long long cost;
  edge() {}
  edge(int _to_, long long _cost_) {
    to = _to_;
    cost = _cost_;
  }
  bool operator>(edge& b) const { return (cost > b.cost); }
};
int parent[1001], Rank[1001];
void Make_set(int node) {
  parent[node] = node;
  Rank[node] = 0;
}
int Find(int node) {
  if (parent[node] == node) return node;
  return parent[node] = Find(parent[node]);
}
bool Union(int a, int b) {
  a = Find(a);
  b = Find(b);
  if (a != b) {
    if (Rank[a] < Rank[b]) swap(a, b);
    parent[b] = a;
    if (Rank[a] == Rank[b]) Rank[a]++;
    return 0;
  }
  return 1;
}
int32_t main() {
  cin.tie(NULL);
  cout.tie(NULL);
  ios_base::sync_with_stdio(false);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    Make_set(i);
  }
  vector<pair<int, int>> closed;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    a--;
    b--;
    if (Union(a, b)) {
      closed.push_back({a, b});
    }
  }
  int a = Find(0);
  cout << closed.size() << '\n';
  for (int i = 0; i < n; i++) {
    if (!Union(a, i)) {
      cout << closed[closed.size() - 1].first << ' '
           << closed[closed.size() - 1].second << ' ' << a + 1 << ' ' << i + 1
           << '\n';
      closed.pop_back();
    }
  }
  return 0;
}
