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 read(int &a) {
  a = 0;
  char c = getchar();
  while (c < '0' || c > '9') {
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    a = (a << 1) + (a << 3) + c - '0';
    c = getchar();
  }
}
int Set(int n, int pos) { return n = n | (1LL << pos); }
int reset(int n, int pos) { return n = n & ~(1LL << pos); }
bool check(int n, int pos) { return (bool)(n & (1LL << pos)); }
int mul(int a, int b) {
  int c;
  c = (a % 1000000007 * b % 1000000007) % 1000000007;
  return c;
}
int add(int a, int b) {
  int c;
  c = (a % 1000000007 + b % 1000000007) % 1000000007;
  return c;
}
int sub(int a, int b) {
  int c;
  c = ((a % 1000000007 - b % 1000000007) % 1000000007 + 1000000007) %
      1000000007;
  return c;
}
int power(int x, int y) {
  int res = 1;
  x = x;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
long long exgcd(long long x, long long y, long long &a, long long &b) {
  int flag = 0;
  long long t, la = 1, lb = 0, ra = 0, rb = 1;
  while (x % y) {
    if (flag == 0)
      la -= x / y * ra, lb -= x / y * rb;
    else
      ra -= x / y * la, rb -= x / y * lb;
    t = x, x = y, y = t % y;
    flag = 1 - flag;
  }
  if (flag == 0)
    a = ra, b = rb;
  else
    a = la, b = lb;
  return y;
}
int n;
vector<pair<int, int> > edges;
int par[1100];
int find_par(int u) {
  if (par[u] == u)
    return u;
  else
    return par[u] = find_par(par[u]);
}
vector<int> pare;
int main() {
  read(n);
  for (int i = 1; i <= n; i++) par[i] = i;
  for (int i = 1; i < n; i++) {
    int u, v;
    read(u);
    read(v);
    int first = find_par(u);
    int second = find_par(v);
    if (first == second)
      edges.push_back(pair<int, int>(u, v));
    else
      par[first] = second;
  }
  for (int i = 1; i <= n; i++) {
    if (par[i] == i) pare.push_back(i);
  }
  if (pare.size() == 1)
    printf("0\n");
  else {
    printf("%d\n", edges.size());
    for (int i = 0, j = 1; i < edges.size() && j < pare.size(); i++, j++) {
      printf("%d %d %d %d\n", edges[i].first, edges[i].second, pare[0],
             pare[j]);
    }
  }
  return 0;
}
