#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int mod = 1000 * 1000 * 1000 + 7;
const int INF = 1e9 + 100;
const ll LINF = 1e18 + 100;
#ifdef DEBUG
#define dbg(x) cout << #x << " = " << (x) << endl << flush;
#define dbgr(s, f) { cout << #s << ": "; for (auto _ = (s); _ != (f); _++) cout << *_ << ' '; cout << endl << flush; }
#else
#define dbg(x) ;
#define dbgr(s, f) ;
#endif
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define fast_io ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define mp make_pair
#define fr first
#define sc second
#define endl '\n'
#define MAXN 500100
int n;
vector<int> adj[MAXN];
vector<pii> ans;
vector<int> leaf;
void dfs(int v, int par)
{
if ((int)adj[v].size() == 1) leaf.pb(v);
for (int u : adj[v])
{
if (u == par) continue;
dfs(u, v);
}
}
int32_t main(void)
{
fast_io;
cin >> n;
FOR(i, 0, n - 1)
{
int x, y;
cin >> x >> y;
adj[x].pb(y);
adj[y].pb(x);
}
int rt = 1;
FOR(i, 1, n + 1) if ((int)adj[i].size() > 1) rt = i;
dfs(rt, -1);
int L = leaf.size();
FOR(i, 0, L/2)
ans.pb({leaf[i], leaf[i + L/2]});
if (L % 2) ans.pb({leaf[L - 2], leaf[L - 1]});
cout << ans.size() << endl;
for (pii p : ans) cout << p.fr << ' ' << p.sc << endl;
return 0;
}
