#include <bits/stdc++.h>
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
using namespace std;
vector<int> leefCount;
vector<int> parent;
vector<vector<int>> neigh;
vector<queue<int>> leefs;
int dfs(int x, int p){
leefCount[x]=(sz(neigh[x])==1);
parent[x]=p;
for (auto u : neigh[x])
{
if(u==p) continue;
leefCount[x]+=dfs(u,x);
}
return leefCount[x];
}
void addL(int x, int p, int indx){
if(sz(neigh[x])==1) leefs[indx].push(x);
for (auto u : neigh[x])
{
if(u==p) continue;
addL(u,x,indx);
}
}
int centroid(int x, int p, int tp){
int tsum=tp;
int mx=tp;
int mxI=0;
for (auto u : neigh[x])
{
if(u==p) continue;
if(leefCount[u]>mx){
mx=leefCount[u];
mxI=u;
}
tsum+=leefCount[u];
}
if(mx*2>tsum) return centroid(mxI,x,tsum-mx);
return x;
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n; cin >> n;
neigh.resize(n);
leefCount.resize(n);
parent.resize(n);
for (int i = 0; i < n-1; i++)
{
int u,v; cin >> u >> v; u--; v--;
neigh[u].push_back(v);
neigh[v].push_back(u);
}
int root=0;
for (int i = 0; i < n; i++)
{
if(sz(neigh[i])>1) { root=i; break; }
}
dfs(root,-1);
root=centroid(root,-1,0);
dfs(root,-1);
sort(all(neigh[root]), [](int a, int b) {
return leefCount[a] < leefCount[b];
});
leefs.resize(sz(neigh[root]));
for (int i = 0; i < sz(neigh[root]); i++)
{
addL(neigh[root][i],root,i);
}
vector<pair<int,int>> ops;
priority_queue<pair<int,int>> pq;
for (int i = 0; i < sz(neigh[root]); i++) pq.push({sz(leefs[i]),i});
while(sz(pq)>1) {
pair<int,int> u=pq.top(); pq.pop();
pair<int,int> v=pq.top(); pq.pop();
ops.push_back({leefs[u.second].front(), leefs[v.second].front()});
leefs[u.second].pop();
leefs[v.second].pop();
u.first--;
v.first--;
if(u.first>0) pq.push(u);
if(v.first>0) pq.push(v);
}
if(sz(pq)==1){
ops.push_back({root,leefs[pq.top().second].front()});
}
cout << sz(ops) << "\n";
for (auto u : ops)
{
cout << u.first+1 << " " << u.second+1 << "\n";
}
return 0;
}
