#include <bits/extc++.h>
using namespace std;
typedef long long ll;
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin>>n;
vector<int> deg(n+1);
for(int i=0;i<n-1;i++){
int a,b;
cin>>a>>b;
deg[a]++;deg[b]++;
}
vector<int> leaves;
for(int i=1;i<=n;i++){
if(deg[i]==1) leaves.emplace_back(i);
}
int l=leaves.size();
int m=l/2;
vector<pair<int,int>> ans;
for(int i=0;i<m;i++){
ans.emplace_back(leaves[i], leaves[i+m]);
}
if(l%2){
ans.emplace_back(leaves[0], leaves[l-1]);
}
cout<<ans.size()<<"\n";
for(auto &p: ans){
cout<<p.first<<" "<<p.second<<"\n";
}
return 0;
}
