#include <bits/stdc++.h>
#define ll long long int
#define f first
#define s second
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pcc pair<char,char>
#define pbb pair<bool,bool>
#define PB push_back
#define PF push_front
#define MOD 1000000007
#define intxt freopen("input.txt","r",stdin)
#define outtxt freopen("output.txt","w",stdout)
using namespace std;
const int MAXN = 5e5 + 10;
int n ,q1 ,q2 ,ans;
vector <int> adj[MAXN];
vector <int> leaf;
inline void dfs(int v , int p = 0) {
bool c = 0;
for(auto u : adj[v]) {
if(u == p) {
continue;
}
dfs(u , v);
c = 1;
}
if(!c) {
ans++;
leaf.PB(v);
}
}
int main() {
ios_base::sync_with_stdio(0);
cin >> n;
for(int i = 1 ; i < n ; i++) {
cin >> q1 >> q2;
adj[q1].PB(q2);
adj[q2].PB(q1);
}
for(int i = 1 ; i <= n ; i++) {
if(adj[i].size() > 1) {
dfs(i);
break;
}
}
if(ans == 1) {
cout << 0 << endl;
return 0;
}
int k = ans / 2;
cout << (ans + 1) / 2 << endl;
for(int i = 0 ; i < k ; i++) {
cout << leaf[i] << " " << leaf[i + k] << endl;
}
if(ans % 2 == 1) {
cout << leaf[leaf.size() - 1] << " " << leaf[0];
}
}
// Hello
