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 int M = 1000000007;
long long int min(long long int a, long long int b, long long int c,
                  long long int d) {
  long long int arr[4] = {a, b, c, d};
  sort(arr, arr + 4);
  return arr[0];
}
long long int max(long long int a, long long int b, long long int c,
                  long long int d) {
  long long int arr[4] = {a, b, c, d};
  sort(arr, arr + 4);
  return arr[3];
}
long long int gcd(long long int a, long long int b) {
  if (b < a) {
    int temp = a;
    a = b;
    b = temp;
  }
  if (a == 0) {
    return b;
  }
  return gcd(b % a, a);
}
long long int lcm(long long int a, long long int b) {
  return (a * b / gcd(a, b));
}
vector<long long int> getprimes() {
  vector<long long int> pl(101, 1);
  for (int i = 2; i <= 100; i++) {
    if (pl[i] == 1) {
      for (int j = 2 * i; j <= 100; j += i) {
        pl[j] = 0;
      }
    }
  }
  return pl;
}
double distsqrd(double x1, double y1, double x2, double y2) {
  return (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
}
long long int fact(long long int first) {
  long long int ans = 1;
  if (first == 0) {
    return 1;
  }
  while (first > 0) {
    ans = (ans * first) % 1000000007;
    first--;
  }
  return ans % 1000000007;
}
long long int power(long long int a, long long int b) {
  long long int ans = 1;
  while (b > 0) {
    if (b % 2 == 1) {
      ans *= a;
      ans = ans;
    }
    b /= 2;
    a *= a;
    a = a;
  }
  return ans;
}
vector<int> getbinary(int first) {
  vector<int> bin(16, 0);
  int iter = 0;
  while (first > 0) {
    if (first % 2 == 1) {
      bin[iter] = 1;
    } else {
      bin[iter] = 0;
    }
    iter++;
    first /= 2;
  }
  return bin;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  map<int, vector<int> > g;
  for (int i = 0; i < n - 1; i++) {
    int u, v;
    cin >> u >> v;
    g[u].push_back(v);
    g[v].push_back(u);
  }
  vector<int> col(n + 1, 0);
  int parent[n + 1];
  vector<int> scc;
  vector<pair<int, int> > nreq;
  stack<int> s;
  for (int i = 1; i <= n; i++) {
    if (col[i] == 0) {
      parent[i] = -1;
      col[i] = 1;
      scc.push_back(i);
      s.push(i);
      while (!s.empty()) {
        int u = s.top();
        s.pop();
        for (auto v : g[u]) {
          if (col[v] == 1) {
            if (v != parent[u]) {
              nreq.push_back(make_pair(u, v));
            }
          } else {
            parent[v] = u;
            col[v] = 1;
            s.push(v);
          }
        }
      }
    }
  }
  n = scc.size();
  cout << n - 1 << endl;
  for (int i = 0; i < n - 1; i++) {
    cout << nreq[i].first << " " << nreq[i].second << " " << scc[i] << " "
         << scc[i + 1] << endl;
  }
}
