#include <bits/stdc++.h>
using namespace std;
int n, s, t, l, r;
vector<int> g[500010];
vector<pair<int, int>> leaf;
void dfs(int no, int pa, int di) {
if(g[no].size() == 1) {
leaf.push_back({di, no});
}
for(auto i: g[no]) {
if(i == pa) continue;
dfs(i, no, di+1);
}
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
for(int i=1; i<n; i++) {
cin >> s >> t;
g[s].push_back(t);
g[t].push_back(s);
}
for(int i=1; i<=n; i++) {
if(g[i].size() > 1) {
dfs(i, -1, 0);
break;
}
}
sort(leaf.begin(), leaf.end());
cout << leaf.size()/2 + leaf.size()%2 << "\n";
for(int i=0; i<leaf.size()/2; i++) {
cout << leaf[i].second << " " << leaf[i + leaf.size()/2 + leaf.size()%2].second << "\n";
}
if(leaf.size()%2 == 1) {
cout << leaf[0].second << " " << leaf[leaf.size()/2].second << "\n";
}
return 0;
}
