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;
long long int parent[10000];
long long int ranc[10000];
vector<pair<long long int, long long int> > v;
long long int makeset(long long int v) {
  parent[v] = v;
  ranc[v] = 0;
}
long long int findset(long long int v) {
  if (parent[v] == v) {
    return parent[v];
  } else
    return findset(parent[v]);
}
long long int unionset(long long int a, long long int b) {
  a = findset(a);
  b = findset(b);
  if (a != b) {
    if (ranc[b] > ranc[a]) swap(a, b);
  }
  parent[b] = a;
  ranc[a] += ranc[b];
}
int main() {
  long long int n;
  cin >> n;
  for (long long int i = 0; i < n; i++) {
    parent[i] = i;
  }
  for (long long int i = 0; i < n - 1; ++i) {
    long long int x;
    long long int y;
    cin >> x >> y;
    if (findset(x) == findset(y)) {
      v.push_back(make_pair(x, y));
    } else
      unionset(x, y);
  }
  cout << v.size() << "\n";
  for (long long int i = 1; i <= n - 1; ++i) {
    for (long long int j = 1; j <= n - 1; ++j) {
      if (i != j) {
        if (findset(i) != findset(j)) {
          cout << v.back().first << " " << v.back().second << " " << i << " "
               << j << "\n";
          v.pop_back();
          unionset(i, j);
        }
      }
    }
  }
  clock_t clk = clock();
  clk = clock() - clk;
  cerr << "Time Elapsed: " << fixed << setprecision(10)
       << ((long double)clk) / CLOCKS_PER_SEC << "\n";
  return 0;
}
