#include<bits/stdc++.h>
using namespace std;
const int N=5e5+5;
int n;
vector<int> adj[N],comp;
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);
}
dfs(1);
comp.emplace_back(1);
int ans=comp.size()/2;
cout << ans << "\n";
for(int i=0;i<ans;i++)cout << comp[i] << " " << comp[i+ans] << "\n";
}
