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.*;
import java.util.*;


public class test {
    int INF = (int)1e9;
    long MOD = 1000000007;
    
    
    int n;
    int[][] g;
    boolean[] vis;
    
    int[] id, sz;
    int[][] o;
    int q;
    
    
    int go(int v, int p, int x) {
        if(vis[v]) return 0;
        vis[v] = true;
//        System.out.println("in go: "+(v+1)+" "+(p+1)+" "+x);
        int rs = 0;
//        System.out.println("for: "+v);
        for(int u=0; u<n; u++) if(g[v][u]>0) {
//            if(u != p) {
                if(!vis[u]) {
                    id[u] = x;
                }
                rs += go(u, v, x) + 1;
//            }
        }
        return rs;
    }
    
    boolean rm(int v, int p) {
        if(vis[v]) return false;
        vis[v] = true;
        for(int u=0; u<n; u++) if(g[v][u]>0) {
            if(u!=p) {
                if(vis[u]) {
                    o[q][0] = v+1;
                    o[q][1] = u+1;
                    g[u][v] = g[v][u] = 0;
                    return true;
                }
                boolean x = rm(u, v);
                if(x) return x;
            }
        }
        return false;
    }
    
    int find(int v) {
        if(id[v] != v)
            id[v] = find(id[v]);
        return id[v];
    }
    
    void union(int v, int u) {
        v = find(v);
        u = find(u);
        vis = new boolean[n];
        if(sz[v]>0) {rm(v, -1); sz[v]--;}
        else {rm(u, -1); sz[u]--;}
        o[q][2] = v+1;
        o[q][3] = u+1;
        q++;
//        System.out.println("new edge: "+(v+1)+" "+(u+1));
        g[u][v] = g[v][u] = 1;
        id[u] = v;
        sz[v] += sz[u];
    }
    
    void solve(InputReader in, PrintWriter out) throws IOException {
        n = in.nextInt();
        g = new int[n][n];
        for(int i=0; i<n-1; i++) {
            int u = in.nextInt()-1;
            int v = in.nextInt()-1;
            g[u][v] = g[v][u] = 1;
        }
        id = new int[n];
        sz = new int[n];
        for(int i=0; i<n; i++) {
            id[i] = i;
            sz[i] = 1;
        }
        q = 0;
        vis = new boolean[n];
        int t = 0;
        for(int i=0; i<n; i++) {
            if(!vis[i]) {
                int rs = go(i, -1, i)/2;
                int l = -t;
                t = 0;
                for(int j=0; j<n; j++) {
                    if(vis[j]) t++;
                }
                l += t;
                id[i] = i;
                sz[i] = rs-l+1;
//                System.out.println("got: "+rs+" "+l);
            }
        }
//        System.out.println(Arrays.toString(id));
//        System.out.println(Arrays.toString(sz));
//        System.out.println("check 1");
        int rs = 0;
        o = new int[n][4];
        q = 0;
        for(int i=0; i<n; i++) {
            for(int j=i+1; j<n; j++) {
                if(find(i) != find(j)) {
                    union(i, j);
//                    System.out.println(Arrays.toString(id));
//                    System.out.println(Arrays.toString(sz));
                }
            }
        }
//        System.out.println("check 2");
        out.println(q);
        for(int i=0; i<q; i++) {
            out.println(o[i][0]+" "+o[i][1]+" "+o[i][2]+" "+o[i][3]);
        }
    }
    
    
    public static void main(String[] args) throws IOException {
        if(args.length>0 && args[0].equalsIgnoreCase("d")) {
            DEBUG_FLAG = true;
        }
        InputReader in = new InputReader();
        PrintWriter out = new PrintWriter(System.out);
        int t = 1;//in.nextInt();
        long start = System.nanoTime();
        while(t-- >0) {
            new test().solve(in, out);
        }
        long end = System.nanoTime();
        debug("\nTime: " + (end-start)/1e6 + " \n\n");
        out.close();
    }
    
    static boolean DEBUG_FLAG = false;
    static void debug(String s) {
        if(DEBUG_FLAG) System.out.print(s);
    }
    
    static class InputReader {
        static BufferedReader br;
        static StringTokenizer st;
    
        public InputReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }
        
        String next() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
        
        int nextInt() {
            return Integer.parseInt(next());
        }
        
        long nextLong() {
            return Long.parseLong(next());
        }
        
        double nextDouble() {
            return Double.parseDouble(next());
        }
    }
}