// chrono::system_clock::now().time_since_epoch().count()
#include <bits/stdc++.h>
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
#define rep(i, a, b) for (int i = (a); i < (b); ++i)
#define debug(x) cerr << #x << " = " << x << endl
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
const int MAXN = (int)5e5 + 5;
vi adj[MAXN], leaf;
int sub[MAXN];
int n, m;
void dfs(int v, int pr = -1) {
sub[v] = (sz(adj[v]) == 1);
for (int to : adj[v]) {
if (pr != to) {
dfs(to, v);
sub[v] += sub[to];
}
}
}
int cen(int v, int lim, int pr = -1) {
for (int to : adj[v]) {
if (to != pr && sub[to] > lim / 2) {
return cen(to, lim, v);
}
}
return v;
}
void dfsCol(int v, int pr, int c) {
if (sz(adj[v]) == 1) {
leaf.pb(v);
}
for (int to : adj[v]) {
if (to != pr) {
dfsCol(to, v, c);
}
}
}
void solve() {
cin >> n;
rep (i, 1, n) {
int u, v;
cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
dfs(1);
int tot = sub[1];
int root = cen(1, tot);
for (int to : adj[root]) {
dfsCol(to, root, ++m);
}
cout << (tot + 1) / 2 << '\n';
if (tot % 2 == 0) {
for (int i = 0; i < tot / 2; i++) {
cout << leaf[i] << ' ' << leaf[i + tot / 2] << '\n';
}
}
else {
for (int i = 0; i <= tot / 2; i++) {
cout << leaf[i] << ' ' << leaf[i + tot / 2] << '\n';
}
}
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int tt = 1;
for (int i = 1; i <= tt; ++i) {
solve();
}
return 0;
}
