#include <iostream>
#include <vector>
using namespace std;
vector<int> nei[1<<19], vec;
void dfs(int u, int p = 0, int ch = 0){
for (int i : nei[u]){
if (i != p)
dfs(i, u), ch++;
}
if (ch == 0)
vec.push_back(u);
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
int n;
cin>>n;
for (int i=1, a, b;i<n;i++){
cin>>a>>b;
nei[a].push_back(b);
nei[b].push_back(a);
}
for (int i=1;i<=n;i++){
if (nei[i].size() > 1)
dfs(i), i = n;
}
n = vec.size();
cout<<(n + 1) / 2<<'\n';
if (n % 2 == 1)
cout<<vec[0]<<" "<<vec[n / 2]<<'\n';
for (int i=0;i < n / 2;i++)
cout<<vec[i]<<" "<<vec[i + (n + 1) / 2]<<'\n';
}
