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 
def sort(component,cycle):
    for i in range(len(component)):
        for j in range(len(component)-1):
            if(len(cycle[component[j]])>len(cycle[component[j+1]])):
                component[j],component[j+1]=component[j+1],component[j]
    return component
def find_set(v,parent):
    if(parent[v]<0):
        return v
    parent[v]=find_set(parent[v],parent)
    return parent[v]
n=int(input())
cycle=[[]for i in range(n+1)]
parent=[-1 for i in range(n+1)]
parent[0]=0
for i in range(n-1):
    x,y=map(int,input().split())
    k=find_set(x,parent)
    b=find_set(y,parent)
    if(k!=b):
        if(parent[k]>=parent[b]):
            parent[k]=b
            parent[b]=parent[b]-1
        else:
            parent[b]=k
            parent[k]=parent[k]-1
    else:
        cycle[k].append((x,y))
t=0;
component=[]
f=[]
for i in range(1,n+1):
        if parent[i]<0:
            t=t+1
            component.append(i)
print(t-1)
component=sort(component,cycle)
i,j=0,len(component)-1
flag=False
while(i<=j and (t-1)!=0):
    if(len(cycle[component[i]])==1):
        flag=True
       
        
    if(flag):
        l,m=cycle[component[i]][0]
        print(l," ",m," ",component[i]," ",component[i+1])
        
    else:
        l,m=cycle[component[j]][0]
        cycle[component[j]].pop(0)
        
        print(l," ",m," ",component[i]," ",component[j])
        if(len(cycle[component[j]])==0):
            j=j-1
    i=i+1

    