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;
int n, u, v, cyclecnt, grno, group[(int)1e3 + 5], vertofgr[(int)1e3 + 5];
bool visited[(int)1e3 + 5];
vector<int> adjlist[(int)1e3 + 5];
pair<int, int> cycles[(int)1e3 + 5];
class UnionFind {
 private:
  int id[(int)1e3 + 5], sz[(int)1e3 + 5];
  int root(int x) {
    while (id[x] != x) {
      id[x] = id[id[x]];
      x = id[x];
    }
    return x;
  }

 public:
  UnionFind() {
    for (int i = 0; i < (int)1e3 + 5; i++) {
      sz[i] = 1;
      id[i] = i;
    }
  }
  bool same(int m, int n) { return root(m) == root(n); }
  void unite(int m, int n) {
    int rtm = root(m), rtn = root(n);
    if (sz[rtm] > sz[rtn]) {
      id[rtn] = id[rtm];
      sz[rtm] += sz[rtn];
    } else {
      id[rtm] = id[rtn];
      sz[rtn] += sz[rtm];
    }
  }
};
inline int fio() {
  int val = 0;
  char ch = getchar();
  while (ch == ' ' || ch == '\n') ch = getchar();
  while (ch >= '0' && ch <= '9') {
    val = val * 10 + ch - '0';
    ch = getchar();
  }
  return val;
}
inline void io() {
  n = fio();
  UnionFind uf;
  for (int i = 0; i < n - 1; i++) {
    u = fio(), v = fio();
    adjlist[u].push_back(v);
    adjlist[v].push_back(u);
    if (!uf.same(u, v))
      uf.unite(u, v);
    else
      cycles[cyclecnt++] = make_pair(u, v);
  }
}
void dfs(int s) {
  visited[s] = true;
  group[s] = grno;
  for (int i = 0; i < adjlist[s].size(); i++) {
    int current = adjlist[s][i];
    if (!visited[current]) {
      dfs(current);
    }
  }
}
inline void mkgroups() {
  for (int i = 1; i <= n; i++)
    if (!visited[i]) {
      grno++;
      vertofgr[grno] = i;
      dfs(i);
    }
}
int main() {
  io();
  mkgroups();
  printf("%d\n", cyclecnt);
  for (int i = 0; i < cyclecnt; i++) {
    printf("%d %d %d %d\n", cycles[i].first, cycles[i].second, vertofgr[1],
           vertofgr[i + 2]);
  }
}
