#include <iostream>
#include <algorithm>
#include <vector>
#include <cassert>
using namespace std;
vector<int> G[500001];
vector<int> leaves;
void dfs(int i, int p) {
if(G[i].size() == 1) leaves.push_back(i);
for(int u: G[i]) {
if(u == p) continue;
dfs(u, i);
}
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
int N; cin >> N;
for(int i = 0; i < N - 1; i++) {
int a, b; cin >> a >> b;
G[a].push_back(b);
G[b].push_back(a);
}
dfs(1, 0);
int ans = (leaves.size() + 1)/2;
cout << ans << endl;
for(int i = 0; i < ans; i++) {
assert((i + leaves.size()/2) < leaves.size());
cout << leaves[i] << " " << leaves[i + leaves.size()/2] << endl;
}
}
