#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef pair<double, double> pdd;
typedef pair<ld , ll> pdl;
typedef string str;
#define all(x) (x).begin(),(x).end()
#define Sort(x) sort(all((x)))
#define X first
#define Y second
#define Mp make_pair
#define pb(x) push_back(x)
#define pf(x) push_front(x)
#define sep ' '
#define endl '\n'
#define debug(x) cerr << #x << " = " << x << endl
#define SZ(x) ll(x.size())
#define fast_io ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
const ll MAXN = 5e5 + 10;
const ll INF = 1e18 + 10;
const ll LOG = 1e7 + 19;
const ll MOD = 998244353;
ll pw(ll a , ll b, ll M) { return (!b ? 1 : (b & 1 ? (a * pw(a * a % M, b / 2, M)) % M : pw(a * a % M, b / 2, M))); }
ll a[MAXN] , mark[MAXN];
vector <ll> adj[MAXN] , barg;
void dfs(ll v){
mark[v] = 1;
if(adj[v].size() == 1){
barg.pb(v);
return;
}
for(auto u: adj[v]){
if(!mark[u]){
dfs(u);
}
}
}
int main(){
fast_io;
ll n;
cin >> n;
for(int i = 1; i < n; i++){
ll v , u;
cin >> v >> u;
adj[v].pb(u);
adj[u].pb(v);
}
ll root = 0;
for(int i = 1; i <= n; i++){
if(adj[i].size() > 1){
root = i;
break;
}
}
dfs(root);
ll k = barg.size() , p = (k + 1) / 2;
k /= 2;
cout << p << endl;
for(int i = 0; i < p; i++){
cout << barg[i] << sep << barg[i + k] << endl;
}
}
