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.IOException;
import java.io.InputStreamReader;


public class RoadsBerland {

	/**
	 * @param args
	 */
	static  int[] ar;
	static int[] size;
	public static void main(String[] args) throws NumberFormatException, IOException {
		// TODO Auto-generated method stub

		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		int[][] extra = new int[n+1][2];
		 ar = new int[n+1];
		
		for(int i=1; i<=n;i++){
			ar[i] = i;
		}
		int index=0;
		 size = new int[n+1];
		for(int i=1;i<n;i++){
			String[] s = br.readLine().split(" ");
		    int v1= Integer.parseInt(s[0]);
		    int v2 = Integer.parseInt(s[1]);
		    
		   if(root(v1)==root(v2)){
			   extra[index][0] = v1;
			   extra[index][1] = v2;
			   index++;
		   }
		   else{
			   union(v1,v2);
			   
		   }
		
		}
		int count=0;
		int pos=0;
		int[] future = new int[n];
		for(int i=1; i<=n;i++){
			if(ar[i]==i){
			
				count++;
				future[pos] = i;
				pos++;
			}
			
		}
		System.out.println(count-1);
		
		for(int i=0; i<=pos-2;i++){
			System.out.println(extra[i][0]+" "+extra[i][1]+" "+future[i]+" "+future[i+1]);
		}
		
	}
	
	private static void union(int x, int y){
		int rx = root(x);
		int ry = root(y);
		if(size[rx]>size[ry]){
			ar[ry] = ar[rx];
		    size[rx]+=size[ry];
		}
		else{
			ar[rx]= ar[ry];
			size[ry]+=size[rx];
		}
	}
	private static int root(int x){
		while(ar[x]!=x){
			ar[x] = ar[ar[x]];
			x = ar[x];
		}
		return x;
	}

}
