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;
int par[2000];
int rank1[2000];
int vis[2000]={0};
vector<vector<int> > g;
vector<pair<int,int> > v;
vector<pair<int,int> > f;

void dfs(int u)
{
	vis[u]=1;
	for(int i=0;i<g[u].size();i++)
	{
		int v=g[u][i];
		if(!vis[v])
		dfs(v);
	}
 } 
int find(int u)
{ 
	if(u==par[u])
	return u;
	else find(par[u]);
}
merge(int x,int y)
{
	if(rank1[x]<rank1[y])
	par[x]=par[y];
	else par[y]=par[x];
	
	if(rank1[x]==rank1[y])
	rank1[y]++;
}
int main()
{
	int n;
	cin>>n;
	g.resize(n);
	int i,j,k,l;
	for(i=0;i<n;i++)
	{
		par[i]=i;
		rank1[i]=-1;
	}
	for(i=0;i<n-1;i++)
	{
		int x;
		int y;
		cin>>x>>y;
		x--;y--;
		int a,b;
		a=find(x);
		par[x]=a;
		b=find(y);
		par[y]=b;
		if(a==b)
		{
			v.push_back(make_pair(x,y));
			continue;
		}
		
		merge(x,y);
		
		g[x].push_back(y);
		g[y].push_back(x);
	
		
	}
	
	k=0;
	for(i=0;i<n;i++)
	{    
	
		if(!vis[i])
		{ f.push_back(make_pair(k,i));
			k=i;
			dfs(i);
		}
	}
	cout<<v.size()<<endl;
	for(i=0;i<v.size();i++)
	{
		cout<<v[i].first+1<<" "<<v[i].second+1<<" "<<f[i+1].first+1<<" "<<f[i+1].second+1<<" ";
	}
}

