#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include <debug.h>
#endif
#define st first
#define nd second
#define pb push_back
#define pf push_front
#define _pb pop_back
#define _pf pop_front
#define lb lower_bound
#define ub upper_bound
#define mtp make_tuple
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
typedef long long ll; typedef unsigned long long ull;
typedef double db; typedef long double ldb;
typedef pair<int, int> pi; typedef pair<ll, ll> pll;
typedef vector<int> vi; typedef vector<ll> vll; typedef vector<pi> vpi; typedef vector<pll> vpll;
typedef string str;
template<typename T> T gcd(T a, T b) { return (b == 0? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
#define FOR(i, l, r) for (int (i) = (l); (i) <= (r); ++(i))
#define FOS(i, r, l) for (int (i) = (r); (i) >= (l); --(i))
#define EACH(i, x) for (auto &(i) : (x))
#define WHILE while
#define file "TEST"
mt19937 rd(chrono::steady_clock::now().time_since_epoch().count());
ll rand(ll l, ll r) { return uniform_int_distribution<ll>(l, r)(rd); }
/*
Tran The Bao
CTL - Da Lat
Cay ngay cay dem nhung deo duoc cong nhan
*/
const int N = 5e5 + 5;
int n, root, cntl[N], cnt = 0;
vi adj[N], a[N], leafs;
vpi ans;
multiset<pi, greater<pi> > s;
void dfs(int u, int p) {
EACH(v, adj[u]) {
if (v == p) continue;
dfs(v, u);
cntl[u] += cntl[v];
}
if (sz(adj[u]) == 1) {
leafs.pb(u);
cntl[u] = 1;
}
}
int fct(int u, int p, int s) {
EACH(v, adj[u]) {
if (v == p || sz(adj[v]) == 1) continue;
if (cntl[v] > s / 2) return fct(v, u, s);
}
return u;
}
void dfs1(int u, int p) {
EACH(v, adj[u]) {
if (v == p) continue;
if (u == root) ++cnt;
dfs1(v, u);
}
if (sz(adj[u]) == 1) a[cnt].pb(u);
}
signed main() {
ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
// freopen(file".inp", "r", stdin);
// freopen(file".out", "w", stdout);
cin >> n;
FOR(i, 1, n - 1) {
int u, v;
cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
FOR(i, 1, n)
if (sz(adj[i]) > 1)
root = i;
dfs(root, 0);
root = fct(root, 0, cntl[root]);
dfs1(root, 0);
FOR(i, 1, cnt) s.insert({sz(a[i]), i});
WHILE(sz(s) > 1) {
int u = (*s.begin()).nd, v = (*++s.begin()).nd;
s.erase(s.begin());
s.erase(s.begin());
ans.pb({a[u].back(), a[v].back()});
a[u]._pb();
a[v]._pb();
if (!a[u].empty()) s.insert({sz(a[u]), u});
if (!a[v].empty()) s.insert({sz(a[v]), v});
}
if (!s.empty()) {
int u = a[(*s.begin()).nd].back();
ans.pb({u, u == leafs[0]? leafs[1] : leafs[0]});
}
cout << sz(ans) << '\n';
EACH(i, ans) cout << i.st << ' ' << i.nd << '\n';
return 0;
}
/*
9
2 1
3 2
4 3
5 4
6 2
7 5
8 5
9 7
*/
