#include <bits/stdc++.h>
#define int long long
int n;
std::vector<int> adj[500010],node[500010];
int leaf[500010],a[500010];
int root=1,cnt=0,cnt2=0;
void get_leaf(int curr,int par){
int sum = adj[curr].size()==1;
for(auto to:adj[curr]){
if(par==to)continue;
get_leaf(to,curr);
sum += leaf[to];
}
leaf[curr]=sum;
}
void get_add(int x, int par){
if(adj[x].size()==1){
node[cnt].push_back(x);
return;
}
for(auto i:adj[x]){
if (i!=par){
get_add(i, x);
}
}
}
int centroid(int curr,int par){
for(auto to:adj[curr]){
if(par==to)continue;
if(leaf[to]>leaf[root]/2)return centroid(to,curr);
}
// if (curr!=par&&leaf[root]-leaf[curr]>leaf[curr]/2)
// return centroid(curr,par);
return curr;
}
signed main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
std::cin >> n;
for(int i=0;i<n-1;i++){
int a,b;
std::cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for(int i=1;i<=n;i++)if(adj[i].size()>1)root=i;
get_leaf(root,root);
int cen=centroid(root,root);
for(auto i:adj[cen]){
++cnt;
get_add(i,cen);
}
for(int i=1;i<=cnt;i++){
for(auto j:node[i]){
a[++cnt2]=j;
}
}
std::cout << (cnt2-1)/2+1<<'\n';
// for(int i=1;i<=cnt2;i++){
// std::cout << a[i];
// if(i%2==1)std::cout << ' ';
// else std::cout << '\n';
// }
// if(cnt2%2==1)std::cout << a[1] << '\n';
for(int i=1;i<=cnt2/2;i++)std::cout <<a[i]<< ' ' << a[i+cnt2/2] << '\n';
if(cnt2%2==1)std::cout << a[cnt2] << ' ' << a[1] << '\n';
}
