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, m, ans, cnt, tot, flag, nu, nv;
struct node {
  int v, next;
} edge[2005];
int head[1005], vis[1005], w[1005], a[1005], num[1005], ok[1005], mp[1005];
bool app[1005][1005];
void addedge(int u, int v) {
  cnt++;
  edge[cnt].v = v;
  edge[cnt].next = head[u];
  head[u] = cnt;
}
void dfs(int u) {
  a[tot]++;
  w[tot] += num[u];
  int i, j, t, v;
  for (i = head[u]; i; i = edge[i].next) {
    v = edge[i].v;
    if (!vis[v]) {
      vis[v] = 1;
      dfs(v);
    }
  }
}
void dfs1(int u, int fa) {
  if (flag) return;
  int i, j, t, v;
  for (i = head[u]; i; i = edge[i].next) {
    v = edge[i].v;
    if (app[u][v]) continue;
    if (!vis[v]) {
      vis[v] = 1;
      dfs1(v, u);
    } else {
      if (v != fa) {
        flag = 1;
        nu = u;
        nv = v;
        return;
      }
    }
  }
}
void solve() {
  int i, j, t;
  tot = 0;
  memset(vis, 0, sizeof(vis));
  memset(w, 0, sizeof(w));
  memset(a, 0, sizeof(a));
  memset(app, 0, sizeof(app));
  ans = 0;
  for (i = 1; i <= n; i++) {
    if (!vis[i]) {
      vis[i] = 1;
      tot++;
      mp[tot] = i;
      dfs(i);
      w[tot] /= 2;
      ans += w[tot] + 1 - a[tot];
    }
  }
  printf("%d\n", ans);
  memset(ok, 0, sizeof(ok));
  for (i = 1; i <= tot; i++) {
    while (w[i] >= a[i]) {
      memset(vis, 0, sizeof(vis));
      vis[mp[i]] = 1;
      flag = 0;
      dfs1(mp[i], 0);
      for (j = 1; j <= tot; j++) {
        if (i == j) continue;
        if (!ok[j]) {
          ok[j] = 1;
          printf("%d %d %d %d\n", nu, nv, nu, mp[j]);
          app[nu][nv] = app[nv][nu] = 1;
          break;
        }
      }
      w[i]--;
    }
  }
}
int main() {
  int i, j, k, t;
  while (~scanf("%d", &n)) {
    int u, v;
    cnt = 0;
    memset(head, 0, sizeof(head));
    memset(num, 0, sizeof(num));
    for (i = 1; i < n; i++) {
      scanf("%d%d", &u, &v);
      num[u]++;
      num[v]++;
      addedge(u, v);
      addedge(v, u);
    }
    solve();
  }
  return 0;
}
