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 md = 1000000007;
long long bexp(long long a, long long b) {
  if (b == 0) return 1;
  if (b == 1) return a;
  long long r = bexp(a, b / 2);
  r = r * r;
  if (b % 2 == 0)
    return r;
  else
    return r * a;
}
long long n, e, m;
vector<long long> ed[2005];
long long par[10005];
long long fd(long long x) {
  if (par[x] < 0) return x;
  return par[x] = fd(par[x]);
}
void un(long long a, long long b) {
  if (abs(par[a]) >= abs(par[b])) {
    par[a] += par[b];
    par[b] = a;
  } else {
    par[b] += par[a];
    par[a] = b;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int test_case = 1;
  while (test_case--) {
    long long i, j, ans = 0, k = 0;
    memset(par, -1, sizeof(par));
    long long l = 0, r = 0, x = 0, y = 0;
    cin >> n;
    for (i = 1; i < n; i++) {
      cin >> l >> r;
      ed[l].push_back(r);
      ed[r].push_back(l);
      x = fd(l);
      y = fd(r);
      if (x != y) {
        un(x, y);
      }
    }
    vector<long long> v;
    long long cnt = 0;
    for (i = 1; i <= n; i++) {
      if (par[i] < 0) {
        v.push_back(i);
        cnt++;
      }
    }
    cout << cnt << "\n";
    for (i = 0; i < cnt; i++) {
      for (j = i + 1; j < cnt; j++) {
        if (ed[v[i]].size() > 0)
          cout << v[i] << " " << ed[v[i]][0] << " " << v[i] << " " << v[j]
               << "\n";
        else if (ed[v[j]].size() > 0)
          cout << v[j] << " " << ed[v[j]][0] << " " << v[j] << " " << v[i]
               << "\n";
      }
    }
  }
  return 0;
}
