#include <bits/stdc++.h>
#define el '\n'
#define fi first
#define sc second
#define int ll
#define pii pair<int, int>
#define all(v) v.begin(), v.end()
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
using namespace std;
using ll=long long;
using ull=unsigned long long;
using ld=long double;
const int mod=1e9+7;
const int N=5e5+11;
int n, root;
vector<int> adj[N], leaf;
void dfs(int u, int pa)
{
if(adj[u].size()==1) leaf.push_back(u);
for(int v:adj[u])
{
if(v!=pa) dfs(v, u);
}
}
void sol()
{
cin >> n;
for(int i=1;i<n;i++)
{
int u, v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for(int i=1;i<=n;i++) if(adj[i].size()>1) root=i;
dfs(root, 0);
int sz=leaf.size();
cout << (sz+1)/2 << el;
for(int i=0; i<(sz+1)/2; i++) cout << leaf[i] << ' ' << leaf[i + sz / 2] << el;
}
signed main()
{
// freopen("divisor.INP", "r", stdin);
// freopen("divisor.OUT", "w", stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);
int t=1;
//cin >> t;
while(t--)
{
sol();
}
}
