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 GCC optimize("O3")
#pragma GCC target("sse4")
using namespace std;
const long double pi = acos(-1);
const long long maxn = 1e3 + 5;
const int mod = 1e9 + 7;
const long long INF = 1e18;
const int MX = 2000000001;
vector<long long> par(maxn), lis[maxn];
vector<pair<long long, long long> > con;
void make_set(long long n) {
  for (long long i = 1; i <= n; i++) {
    par[i] = i;
    lis[i] = vector<long long>(1, i);
  }
}
long long find_par(long long n) { return par[n]; }
void union_set(long long a, long long b) {
  long long prea = a, preb = b;
  a = find_par(a), b = find_par(b);
  if (a != b) {
    if ((long long)(lis[a]).size() < (long long)(lis[b]).size()) swap(a, b);
    while (!lis[b].empty()) {
      long long x = lis[b].back();
      lis[b].pop_back();
      par[x] = a;
      lis[a].push_back(x);
    }
  } else
    con.push_back(make_pair(prea, preb));
}
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0);
  long long t, q, n, a, b, c, d, k, l, m, r, x = 0, y = 0, z = 0, sum = 0,
                                             ans = 0, temp = 0, res = 0;
  cin >> n;
  make_set(n);
  for (long long i = 1; i < (n); i++) {
    cin >> a >> b;
    union_set(a, b);
  }
  unordered_set<long long> st;
  for (long long i = 1; i <= n; i++) st.insert(par[i]);
  cout << (long long)(st).size() - 1 << "\n";
  if ((long long)(st).size() == 1) {
    return 0;
  }
  auto itr = st.begin();
  itr++;
  for (auto x = itr; x != st.end(); x++) {
    cout << con.front().first << " " << con.front().second << " " << *st.begin()
         << " " << *x << "\n";
    con.erase(con.begin());
  }
}
