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 
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Random;

public class D {
    public static void main(String[] args) throws Exception {
        Random rand = new Random();
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        int[] par = new int[n];
        for (int i = 0; i < n; ++i)
            par[i] = i;
        ArrayList<Integer> del = new ArrayList<Integer>();
        for (int z = 0; z < n - 1; ++z) {
            String[] ss = br.readLine().split(" ");
            int i = Integer.parseInt(ss[0]);
            int j = Integer.parseInt(ss[1]);
            --i; --j;
            if (getParent(i, par) == getParent(j, par)) {
                del.add(i);
                del.add(j);
            } else {
                if (rand.nextBoolean())
                    par[par[i]] = par[j];
                else
                    par[par[j]] = par[i];
            }
        }
        System.out.println(del.size() / 2);
        int at = 0;
        for (int i = 0; i < n; ++i)
            for (int j = i + 1; j < n; ++j)
                if (getParent(i, par) != getParent(j, par)) {
                    if (rand.nextBoolean())
                        par[par[i]] = par[j];
                    else
                        par[par[j]] = par[i];
                    System.out.println((del.get(at) + 1) + " " + (del.get(at + 1) + 1) + " " + (i + 1) + " " + (j + 1));
                    at += 2;
                }
        assert(at == del.size());
    }

    private static int getParent(int i, int[] p) {
        return p[i] == i ? i : (p[i] = getParent(p[i], p));
    }
}
