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 
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;

typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;

typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ld, ld> pd;

typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;

template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define FOR(i, a, b) for (int i=a; i<(b); i++)
#define F0R(i, a) for (int i=0; i<(a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= a; i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)

#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define fr first
#define se second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()

mt19937 rng (chrono::steady_clock::now().time_since_epoch().count() );

const int MOD = 998244353;
const ll INF = 1e18;
const int MX = 2e5 + 12; //check the limits, dummy

vl v[MX];
map<pl, bool>bad;
bool vis[MX];
vpl not_needed, needed;

void dfs (ll src, ll par) {

    vis[src] = 1;
    for (auto w : v[src]) {

        if (vis[w] == 1) {
            if (w != par && bad[ {src, w}]) not_needed.pb ({src, w});
            else if (w != par && bad[ {w, src}]) not_needed.pb ({w, src});

            bad[ {src, w}] = 1;
            bad[ {w, src}] = 1;
        } else {
            dfs (w, src);
        }

    }
}


int main() {

    ios_base::sync_with_stdio (0);
    cin.tie (0);

    ll n;
    cin >> n;
    FOR (i, 0, n - 1) {
        ll a, b;
        cin >> a >> b;
        v[a].pb (b);
        v[b].pb (a);
    }

    ll extra_edge = -1;
    FOR (i, 1, n + 1) {
        if (vis[i] == 0) {

            if (extra_edge != -1) needed.pb ({extra_edge, i});

            ll x = sz (not_needed);
            dfs (i, -1);
            if (x != sz (not_needed) ) {
                extra_edge = i;
            }
        }
    }

    cout << sz (needed) << '\n';
    FOR (i, 0, sz (needed) ) cout << not_needed[i].fr << ' ' << not_needed[i].se << ' ' << needed[i].fr << ' ' << needed[i].se << '\n';
    return  0;
}
