#include<bits/stdc++.h>
using namespace std;
const int N=5e5+5;
int n;
int sz[N];
vector<int> adj[N],comp;
int dfssz(int u,int p=0){
sz[u]=(adj[u].size()<2);
for(auto v:adj[u])if(v!=p)sz[u]+=dfssz(v,u);
return sz[u];
}
int centroid(int u,int cnt,int p=0){
for(auto v:adj[u])if(v!=p&&sz[v]>cnt/2)return centroid(v,cnt,u);
return u;
}
void dfs(int u,int p=0){
if(adj[u].size()==1)comp.emplace_back(u);
for(auto v:adj[u])if(v!=p)dfs(v,u);
}
int main(){
cin.tie(nullptr)->sync_with_stdio(false);
cin >> n;
for(int i=1;i<n;i++){
int u,v;
cin >> u >> v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
int c=centroid(1,dfssz(1));
dfs(c);
comp.emplace_back(c);
int ans=comp.size()/2;
cout << ans << "\n";
for(int i=0;i<ans;i++)cout << comp[i] << " " << comp[i+ans] << "\n";
}
