#include<bits/stdc++.h>
using namespace std;
vector<int> l;
vector<vector<int>> g;
void dfs(int u, int p) {
if (g[u].size()==1) l.push_back(u);
for (auto v : g[u]) {
if (v==p) continue;
dfs(v, u);
}
}
int main() {
cin.tie(0)->sync_with_stdio(false);
int n, k;
cin >> n;
g.assign(n+1, vector<int>());
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, 1);
cout << (k=l.size()+1)/2 << '\n';
for (int i = 0; i < k/2; ++i) {
cout << l[i] << ' ' << l[i+(k-1)/2] << '\n';
}
return 0;
}
