#include<bits/stdc++.h>
using namespace std;
vector<int>adj[500005];
int sz[500005];
vector<int>v;
int dfssz(int u,int p){
if(adj[u].size()==1)sz[u]=1;
for(auto x:adj[u])if(x!=p)sz[u]+=dfssz(x,u);
return sz[u];
}
int centroid(int u,int p,int rsz){
for(auto x:adj[u])if(x!=p&&sz[x]*2>rsz)return centroid(x,u,rsz);
return u;
}
void dfs(int u,int p){
if(adj[u].size()==1)v.push_back(u);
for(auto x:adj[u])if(x!=p)dfs(x,u);
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;cin>>n;
for(int i=1;i<n;i++){
int a,b;cin>>a>>b;
adj[a].push_back(b);
adj[b].push_back(a);
}
int rt=0;
dfs(rt=centroid(1,0,dfssz(1,0)),0);
vector<pair<int,int>>ans;
for(int i=0;i<v.size()/2;i++){
int add=v.size()-(v.size()/2);
ans.push_back({v[i],v[i+add]});
}
if(v.size()%2==1)ans.push_back({v[v.size()/2],rt});
cout<<ans.size()<<'\n';
for(auto x:ans)cout<<x.first<<' '<<x.second<<"\n";
}
