#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define eb emplace_back
#define pu push
#define ins insert
#define fi first
#define se second
#define all(a) a.begin(),a.end()
#define bruh ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define fu(x,a,b) for (auto x=a;x<=b;x++)
#define fd(x,a,b) for (auto x=a;x>=b;x--)
#define int ll
using namespace std;
//mt19937 mt(chrono::steady_clock::now().time_since_epoch().count());
/*
Competitive Programming notes that I need to study & fix my dumbass self:
1. Coding:
- Always be sure to check the memory of arrays (maybe use vectors), for loops
- Always try to maximize the memory if possible, even if you are going for subtasks
- Do not exploit #define int long long, it will kill you
2. Stress:
- Always try generating big testcases and try if they run
3. Time management:
- Don't overcommit or undercommit, always spend a certain amount of time to think a problem, don't just look at it and say I'm fucked
- Do not spend too much time coding brute-force solutions, they should be easily-codable solutions that don't take up too much time
Time management schedule:
Offline / LAH days (4 problems - 3h):
15' thinking of solution / idea
1. no idea: skip
2. yes idea: continue thinking for <= 15'
+ implementing: <= 20'
+ brute-force: <= 5'
+ test generator: <= 5'
I hate offline because I am dumb
*/
typedef pair<int, int> ii;
const int N = 5e5+5;
const int B = 750;
const int mod = 1e9+7;
const int inf = 1e18;
using cd = complex<double>;
const long double PI = acos(-1);
int power(int a,int b) {ll x = 1;if (a >= mod) a%=mod; while (b) {if (b & 1) x = x*a % mod;a = a*a % mod;b>>=1;}return x;}
int n;
vector<int> adj[N];
vector<int> leaf;
int tin[N], timer = 0;
void dfs(int u, int p)
{
tin[u] = ++timer;
if (adj[u].size() == 1) leaf.pb(u);
for (auto v : adj[u])
{
if (v == p) continue;
dfs(v, u);
}
}
bool cmp(int i, int j)
{
return tin[i] < tin[j];
}
void solve()
{
cin>>n;
for (int i = 1; i < n; i++)
{
int u,v; cin>>u>>v;
adj[u].pb(v); adj[v].pb(u);
}
int root;
for (int i = 1; i <= n; i++) if (adj[i].size() > 1) root = i;
dfs(root, 0);
sort(all(leaf), cmp);
vector<ii> ans;
int m = leaf.size();
for (int i = 0; i < m/2; i++) ans.pb({leaf[i], leaf[i+m/2]});
if (m % 2 == 1) ans.pb({leaf[0], leaf[m-1]});
cout<<ans.size()<<endl;
for (auto i : ans) cout<<i.fi<<" "<<i.se<<endl;
}
/*
Go through the mistakes you usually make and revise your code, for god's sake...
*/
signed main()
{
bruh
//freopen("input.inp","r",stdin);
//freopen("output.inp","w",stdout);
int t = 1;
// cin>>t;
while (t--)
{
solve();
cout<<"\n";
}
}
