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.util.Scanner;

public class Main {
    static int posDest = 0;
    static int posAdd = 0;
    static Pair[] dest = new Pair[2000];
    static Pair[] add = new Pair[2000];

    static class Pair {
        int f;
        int s;

        Pair(int f, int s) {
            this.f = f;
            this.s = s;
        }
    }

    static class DSU {
        int[] size;
        int[] parent;

        DSU(int n) {
            size = new int[n+1];
            parent = new int[n+1];
            for(int i = 1; i <= n; i++) {
                size[i] = 1;
                parent[i] = i;
            }
        }

        int findSet(int val) {
            if(parent[val] == val) {
                return val;
            }
            parent[val] = findSet(parent[val]);
            return parent[val];
        }

        boolean joinSet(int a, int b) {
            int fa = a;
            int fb = b;

            a = findSet(a);
            b = findSet(b);

            if(a == b) {
                dest[posDest++] = new Pair(fb, fa);
                return false;
            }

            if(size[a] < size[b]) {
                int aux = a;
                a = b;
                b = aux;
            }

            size[a] += size[b];
            parent[b] = a;
            return true;
        }
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        DSU dsu = new DSU(n);
        for(int i = 0; i < n - 1; i++) {
            int fi = sc.nextInt();
            int si = sc.nextInt();
            dsu.joinSet(fi, si);
        }
        for(int i = 2; i <= n; i++) {
            if(dsu.joinSet(1, i)) {
                add[posAdd++] = new Pair(1, i);
            }
        }
        System.out.println(posAdd);
        for(int i = 0; i < posAdd; i++) {
            System.out.println(dest[i].f + " " + dest[i].s + " " + add[i].f + " " + add[i].s);
        }
    }
}