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;
void optimise() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
}
long long int mod = (long long int)1e9 + 7;
void normalize(long long int &a) { a = (a + mod) % mod; }
long long int modmul(long long int a, long long int b) {
  a = a % mod;
  b = b % mod;
  normalize(a);
  normalize(b);
  return (a * b) % mod;
}
long long int modadd(long long int a, long long int b) {
  a = a % mod;
  b = b % mod;
  normalize(a);
  normalize(b);
  return (a + b) % mod;
}
long long int modsub(long long int a, long long int b) {
  a = a % mod;
  b = b % mod;
  normalize(a);
  normalize(b);
  return (a - b + mod) % mod;
}
long long int me(long long int x, long long int n) {
  x %= mod;
  if (n == 0) return 1;
  long long int u = me(x, n / 2) % mod;
  u = (u * u) % mod;
  if (n % 2) u = (u * x) % mod;
  return u;
}
long long int me1(long long int x, long long int n) {
  if (n == 0) return 1;
  long long int u = me1(x, n / 2);
  u = u * u;
  if (n % 2) u = u * x;
  return u;
}
inline long long int modInv(long long int a) { return me(a, mod - 2); }
inline long long int modDiv(long long int a, long long int b) {
  return modmul(a, modInv(b));
}
float power(float x, long long int y) {
  float temp;
  if (y == 0) return 1;
  temp = power(x, y / 2);
  if (y % 2 == 0)
    return temp * temp;
  else {
    if (y > 0)
      return x * temp * temp;
    else
      return (temp * temp) / x;
  }
}
long long int __gcd(long long int a, long long int b) {
  if (b == 0)
    return a;
  else
    return __gcd(b, a % b);
}
pair<long long int, long long int> operator+(
    pair<long long int, long long int> a,
    pair<long long int, long long int> b) {
  return {a.first + b.first, a.second + b.second};
}
pair<long long int, long long int> operator-(
    pair<long long int, long long int> a,
    pair<long long int, long long int> b) {
  return {a.first - b.first, a.second - b.second};
}
pair<long long int, long long int> operator*(
    pair<long long int, long long int> a,
    pair<long long int, long long int> b) {
  return {a.first * b.first, a.second * b.second};
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &v) {
  for (auto it : v) os << it << " ";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  for (long long int i = 0; i < v.size(); ++i) os << v[i] << " ";
  return os;
}
const long long int N = 1e3 + 1;
vector<long long int> vis(N);
vector<long long int> G[N];
vector<long long int> parent(N);
map<pair<long long int, long long int>, long long int> s;
long long int g;
void dfs(long long int v, long long int p) {
  vis[v] = 1;
  for (auto y : G[v]) {
    if (vis[y] and p != y and s.find({v, y}) == s.end() and
        s.find({y, v}) == s.end()) {
      s[{v, y}] = g;
    } else if (vis[y] == 0) {
      dfs(y, v);
    }
  }
}
void solve() {
  long long int n;
  cin >> n;
  vector<long long int> size1(n + 1);
  for (long long int i = 0; i < n - 1; i++) {
    long long int x, y;
    cin >> x >> y;
    G[x].push_back(y);
    G[y].push_back(x);
  }
  set<long long int> ans;
  for (long long int i = 1; i <= n; i++) {
    if (vis[i] == 0) {
      g = i;
      dfs(i, -1);
      if (i != 1) ans.insert(i);
    }
  }
  vector<pair<pair<long long int, long long int>,
              pair<long long int, long long int> > >
      p1;
  while (!ans.empty()) {
    auto k1 = *s.begin();
    auto k2 = *ans.begin();
    p1.push_back({{k1.first}, {k2, 1}});
    s.erase(s.begin());
    ans.erase(k2);
  }
  cout << p1.size() << "\n";
  for (long long int i = 0; i < p1.size(); i++) {
    cout << p1[i].first.first << " " << p1[i].first.second << " "
         << p1[i].second.first << " " << p1[i].second.second << "\n";
  }
}
signed main() {
  optimise();
  long long int t;
  t = 1;
  cout << fixed << setprecision(3);
  while (t--) {
    solve();
    cout << "\n";
  }
  cerr << "\n"
       << "Time Elasped : " << 1.0 * clock() / CLOCKS_PER_SEC << "\n";
}
