#include<bits/stdc++.h>
using namespace std;
#define INIT ios_base :: sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define mp make_pair
#define pb push_back
#define ft first
#define sc second
#define ll long long
#define pii pair<int, int>
#define count_bits __builtin_popcount
//#define int ll
int t, n, m, k, q, l, r;
vector<int> g[500010];
vector<int> leaves;
bool v[500010];
void dfs(int s){
v[s]=true;
if(g[s].size()==1){
leaves.pb(s);
}
for(auto u:g[s]){
if(!v[u]){
dfs(u);
}
}
}
int32_t main(){
INIT
cin>>n;
for(int i=1; i<n; i++){
int a, b;
cin>>a>>b;
g[a].pb(b); g[b].pb(a);
}
dfs(1);
int sz=((int)leaves.size()+1)>>1ll, add=((int) leaves.size())>>1ll ;
cout<<sz<<"\n";
for(int i=0; i<sz; i++){
cout<<leaves[i]<<" "<<leaves[i+add]<<"\n";
}
return 0;
}
