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 tc, node, edge, par[1005], szz[1005];
vector<long long int> graph[1005];
vector<pair<long long int, long long int> > ex[1005];
struct cs {
  long long int x;
  long long int y;
  long long int u;
  long long int v;
};
long long int find_par(long long int r) {
  if (par[r] == r) {
    return r;
  } else
    return par[r] = find_par(par[r]);
}
void union_find(long long int x, long long int y) {
  long long int u = find_par(x);
  long long int v = find_par(y);
  if (u != v) {
    par[v] = u;
    for (int i = 0; i < ex[v].size(); i++) {
      ex[u].push_back(ex[v][i]);
    }
    ex[v].clear();
  } else {
    ex[u].push_back(make_pair(x, y));
  }
}
void mems(long long int n) {
  for (int i = 1; i <= n; i++) {
    par[i] = i;
    szz[i] = 0;
    graph[i].clear();
    ex[i].clear();
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  while (cin >> node) {
    mems(node);
    for (int i = 1; i <= node - 1; i++) {
      long long int x, y;
      cin >> x >> y;
      graph[x].push_back(y);
      graph[y].push_back(x);
      union_find(x, y);
    }
    long long int cnt = 0;
    vector<cs> ans;
    cs ss;
    for (int i = 1; i <= node; i++) {
      if (ex[i].size() > 0) {
        long long int pi = find_par(i);
        for (int k = 1; k <= node && ex[i].size() > 0; k++) {
          long long int pk = find_par(k);
          if (pk != pi) {
            long long int sz = ex[i].size();
            ex[i].pop_back();
            ss.x = ex[i][sz - 1].first;
            ss.y = ex[i][sz - 1].second;
            ss.u = i;
            ss.v = k;
            ans.push_back(ss);
            cnt++;
            par[pk] = pi;
          }
        }
      }
    }
    if (cnt == 0) {
      cout << "0" << endl;
    } else {
      cs ss;
      cout << cnt << endl;
      for (int i = 0; i < ans.size(); i++) {
        ss = ans[i];
        cout << ss.x << ' ' << ss.y << ' ' << ss.u << ' ' << ss.v << endl;
      }
    }
  }
  return 0;
}
