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 
n = int( input() )

p = [0]*n
rank = [0]*n
deg = [0]*n
edges = []

for i in range( n ):
    p[i] = i

def find_set( x ):

    if x == p[x]:
        return ( x )

    return ( find_set( p[x] ) )

def union_set( a, b ):

    fa = find_set( a )
    fb = find_set( b )

    if rank[fa] < rank[fb]:
        p[fa] = fb
    else:
        p[fb] = fa

    if rank[a] == rank[b]:
        rank[a] += 1

adj = [ ]
for i in range(n):
    adj.append([])

memo = {}

for i in range( n-1 ):
    a, b = list( map( int, input().split() ) )
    deg[a-1] += 1
    deg[b-1] += 1
    adj[a-1].append(b-1)
    adj[b-1].append(a-1)
    memo[ (a,b) ] = 1
    memo[ (b,a) ] = 1
    union_set( a-1, b-1 )

visited = [False]*n
group = [0]*n

def dfs( u, c ):

    if visited[u]:
        return

    visited[u] = True
    group[u] = c

    for v in adj[u]:
        if visited[v] == False:
            dfs( v, c )

cnt = 0
for i in range(n):
    if visited[i] == False:
        cnt += 1
        dfs(i, cnt)

#print( group )

P = []

for i in range(n):
    if find_set(i) == i:
        for j in range(n):
            if i != j and find_set(j) == j:

                fl = False
                idx = -1
                for v in adj[i]:
                    if fl == False:
                        fl = True
                        idx = v
                    elif deg[idx] < deg[v]:
                        idx = v

                P.append( (i+1, idx+1, i+1, j+1) )
                union_set( i, j )

print( len(P ) )

for i, j, k, z in P:
    print( i, j, k, z )
