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:16777216")
using namespace std;
vector<vector<pair<long long, char>>> g;
long long n;
vector<char> cl;
vector<long long> p;
vector<pair<long long, bool>> comp;
long long cycle_st, cycle_end;
vector<char> u;
void bfs(long long s) {
  queue<long long> q;
  q.push(s);
  u[s] = true;
  while (!q.empty()) {
    long long v = q.front();
    q.pop();
    for (long long i = 0; i < g[v].size(); ++i) {
      if (!(g[v][i].second)) continue;
      long long to = g[v][i].first;
      if (!u[to]) {
        u[to] = true;
        q.push(to);
      }
    }
  }
}
bool dfs(long long v) {
  cl[v] = 1;
  for (long long i = 0; i < g[v].size(); ++i) {
    if (!(g[v][i].second)) continue;
    long long to = g[v][i].first;
    if (cl[to] == 0) {
      p[to] = v;
      if (dfs(to)) return true;
    } else if (cl[to] == 1) {
      cycle_end = v;
      cycle_st = to;
      return true;
    }
  }
  cl[v] = 2;
  return false;
}
int main() {
  cin >> n;
  g.resize(n + 1);
  p.resize(n + 1);
  cl.resize(n + 1);
  u.resize(n + 1, false);
  long long x, y;
  for (long long i = 0; i < n - 1; ++i) {
    cin >> x >> y;
    g[x].push_back(make_pair(y, true));
    g[y].push_back(make_pair(x, true));
  }
  for (long long i = 1; i <= n; ++i) {
    if (!u[i]) {
      bfs(i);
      comp.push_back(make_pair(i, true));
    }
  }
  long long go = comp.size() - 1;
  if (!go) {
    cout << 0;
    return 0;
  }
  long long w = 0;
  vector<pair<pair<long long, long long>, pair<long long, long long>>> ans;
  for (;;) {
    if (!go) break;
    if (!comp[w].second) continue;
    if (dfs(comp[w].first)) {
      pair<long long, long long> e = make_pair(cycle_end, p[cycle_end]);
      for (long long k = 0; k < g[e.first].size(); ++k) {
        if (g[e.first][k].first == e.second) g[e.first][k].second = false;
      }
      for (long long k = 0; k < g[e.second].size(); ++k) {
        if (g[e.second][k].first == e.first) g[e.second][k].second = false;
      }
      for (long long j = 0; j < comp.size(); ++j) {
        if (comp[j].second && j != w) {
          comp[j].second = false;
          g[comp[j].first].push_back(make_pair(e.first, true));
          g[e.first].push_back(make_pair(comp[j].first, true));
          ans.push_back(make_pair(make_pair(e.first, e.second),
                                  make_pair(comp[j].first, e.first)));
          break;
        }
      }
      p.assign(p.size(), 0);
      cl.assign(cl.size(), 0);
      --go;
    } else
      ++w;
  }
  cout << ans.size() << endl;
  for (long long i = 0; i < ans.size(); ++i) {
    cout << ans[i].first.first << ' ' << ans[i].first.second << ' '
         << ans[i].second.first << ' ' << ans[i].second.second << endl;
  }
}
