#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3,unroll-loops")
using namespace std;
#define int long long
#define pii pair<int,int>
#define ff first
#define ss second
#define sp << " " <<
#define all(cont) cont.begin(),cont.end()
#define vi vector<int>
const int inf = 1e17,N = 5e5+1,MOD = 1e9+7,BL = 1000;
vi edges[N];
vi leaf;
void leadfs(int node,int p) {
for (auto it : edges[node]) {
if (it == p) continue;
leadfs(it,node);
}
if (edges[node].size() == 1) leaf.push_back(node);
}
void solve() {
int n;
cin >> n;
for (int i=1;i<n;i++) {
int a,b;
cin >> a >> b;
edges[a].push_back(b);
edges[b].push_back(a);
}
int root = 1;
for (int i=1;i<=n;i++) {
if (edges[i].size() == 1) continue;
root = i;
}
leadfs(root,root);
cout << (leaf.size()+1)/2 << '\n';
for (int i=0;i<leaf.size()/2;i++) {
cout << leaf[i] sp leaf[i+leaf.size()/2] << '\n';
}
if (leaf.size()%2) cout << leaf[0] sp leaf.back() << '\n';
}
int32_t main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef Dodi
freopen("in.txt","r",stdin);
freopen("out.txt","w",stdout);
#endif
int t = 1;
//cin >> t;
while (t --> 0) solve();
}
