#include <bits/stdc++.h>
using namespace std;
int n, a, b;
vector<vector<int> > graph;
vector<int> L;
void dfs(int u, int p) {
for(int &v : graph[u]) {
if(v == p) continue;
dfs(v, u);
}
if(graph[u].size() == 1) L.push_back(u);
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n;
graph.resize(n+1);
for(int i=0; i<n-1; i++) {
cin >> a >> b;
graph[a].push_back(b);
graph[b].push_back(a);
}
dfs(1, 0);
int sz = L.size();
cout << (sz + 1) / 2 << '\n';
for(int i=0; i<sz/2; i++)
cout << L[i] << " " << L[i+sz/2] << '\n';
if(sz % 2 == 1) cout << L[0] << " " << L.back() << '\n';
return 0;
}
