#include <bits/stdc++.h>
using namespace std;
vector<int> graph[500001], leaves;
void dfs(int node = 1, int parent = 0) {
if (graph[node].size() == 1) leaves.push_back(node);
for (int i : graph[node]) {
if (i == parent) continue;
dfs(i, node);
}
}
int main() {
int n;
cin >> n;
for (int i = 0; i < n - 1; i++) {
int a, b;
cin >> a >> b;
graph[a].push_back(b);
graph[b].push_back(a);
}
dfs();
cout << (leaves.size() + 1) / 2 << '\n';
for (int i = 0; i + leaves.size() / 2 < leaves.size(); i++) cout << leaves[i] << ' ' << leaves[i + leaves.size() / 2] << '\n';
return 0;
}
