#include <bits/stdc++.h>
using namespace std;
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define sz(a) a.size()
typedef long long ll;
typedef pair<ll, ll> pii;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<class T> using oset=tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define int long long
vector<int> adj[500005];
bool isLeaf[500005];
set<int> leafs[500005];
vector<pii> links;
int root = -1;
int N;
void dfs(int v, int p){
//cout << v << endl;
assert(v >= 0 && v < N);
for(int u : adj[v]){
if(u == p)continue;
isLeaf[v] = false;
dfs(u,v);
}
if(isLeaf[v]){
//cout << v << '\n';
leafs[v].insert(v);
}
vector<int> one;
vector<int> two;
map<int, int> imp;
for(int u : adj[v]){
if(u == p)continue;
if(leafs[u].size() == 1)one.pb(u);
if(leafs[u].size() == 2)two.pb(u);
if(v == root){
imp[u] = *leafs[u].begin();
}
assert(leafs[u].size() == 1 || leafs[u].size() == 2);
for(int x : leafs[u]){
leafs[v].insert(x);
}
}
//cout << v << " " << leafs[v].size() << " " << (int)leafs[v].size()-2 << endl;
//cout << v << " " << one.size() << " " << two.size() << "\n";
while((ll)leafs[v].size() - 2 >= 1LL){
//cout << "hi\n";
assert(leafs[v].size() == one.size() + 2 * two.size());
if(!two.empty()){
int tw = two[two.size()-1];
two.pop_back();
int o = -1;
if(!two.empty()){
o = two[two.size()-1];
two.pop_back();
one.push_back(o);
}
else{
assert(!one.empty());
o = one[one.size()-1];
one.pop_back();
}
one.push_back(tw);
assert(o != -1 && tw != o);
//cout << tw << " " << o << '\n';
//cout << *leafs[tw].begin() << " " << *leafs[o].begin() << '\n';
//cout << *leafs[tw].begin() << " " << *(--leafs[tw].end()) << '\n';
//link with back of one
links.pb({*leafs[tw].begin(), *leafs[o].begin()});
leafs[v].erase(*leafs[tw].begin());
leafs[v].erase(*leafs[o].begin());
leafs[tw].erase(*leafs[tw].begin());
leafs[o].erase(*leafs[o].begin());
}
else{
assert(one.size() >= 2);
int ba = one[one.size()-1];
one.pop_back();
int ba2 = one[one.size()-1];
one.pop_back();
assert(ba != ba2);
links.pb({*leafs[ba].begin(), *leafs[ba2].begin()});
leafs[v].erase(*leafs[ba].begin());
leafs[v].erase(*leafs[ba2].begin());
leafs[ba].erase(*leafs[ba].begin());
leafs[ba2].erase(*leafs[ba2].begin());
}
}
if(v == root){
//remove the final ones
assert(p == -1);
assert(leafs[v].size() == 1 || leafs[v].size() == 2 || leafs[v].size() == 0);
if(leafs[v].size() == 0){
assert(one.size() == 0 && two.size() == 0);
return;
}
else if(leafs[v].size() == 2){
assert(one.size() == 2 && two.size() == 0);
assert(one[0] != one[1]);
links.pb({*leafs[v].begin(), *(--leafs[v].end())});
}
else{
//cout << *leafs[one[0]].begin() << '\n';
assert(one.size() == 1 && two.size() == 0);
for(int u : adj[v]){
if(u != p && u != one[0]){
//cout << u << '\n';
//cout << *leafs[u].begin() << '\n'
assert(imp[u] != *leafs[one[0]].begin());
links.pb({imp[u], *leafs[one[0]].begin()});
break;
}
}
}
}
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> N;
for(int i =0;i < N-1; i++){
int a, b;
cin >> a >> b;
isLeaf[i]=true;
a--;
b--;
adj[a].pb(b);
adj[b].pb(a);
}
isLeaf[N-1]=true;
for(int i = 0; i < N; i++){
if(adj[i].size() > 1){
root = i;
isLeaf[root] = false;
break;
}
}
assert(root != -1);
dfs(root, -1);
int leafs = 0;
for(int i = 0; i < N; i++){
if(isLeaf[i])leafs++;
}
cout << links.size() << '\n';
assert((ll)links.size() == (1LL * leafs+1)/2);
for(int i = 0; i < (int)links.size(); i++){
assert(links[i].f != links[i].s);
assert(links[i].f >= 0 && links[i].f < N);
assert(links[i].s >= 0 && links[i].s < N);
cout << links[i].f + 1 << " " << links[i].s + 1 << '\n';
}
}
