#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define dd(x) cout<<#x<<" is "<<x<<endl;
#define dd2(x,y) cout<<#x<<" is "<<x<<" "<<#y<<" is "<<y<<endl;
#define dl(x) cout<<#x<<" is "<<endl; for(auto i:x) cout<<i<<' '; cout<<endl;
#define fi first
#define se second
typedef pair<int,int> pii;
const int N = 5e5+5;
vector<int>adj[N];
int st[N];
int cnt = 1;
void dfs(int x, int p){
st[x] = cnt++;
for(auto i: adj[x]){
if(i == p) continue;
dfs(i,x);
}
}
bool cmp(int a, int b){
return st[a] < st[b];
}
signed main(){
ios::sync_with_stdio(0); cin.tie(0);
int n; cin>>n;
for(int i=0; i<n-1; i++){
int a,b; cin>>a>>b;
adj[a].pb(b);
adj[b].pb(a);
}
int d = 0, root = -1;
vector<int>leaves;
for(int i=1; i<=n; i++) {
if(adj[i].size() == 1) d++, leaves.pb(i);
else root = i;
}
dfs(root,-1);
sort(leaves.begin(), leaves.end(), cmp);
cout<<(d+1)/2<<'\n';
for(int i=0; i<(d+1)/2; i++){
cout<<leaves[i]<<' '<<leaves[i+(d/2)]<<'\n';
}
return 0;
}
