#include <bits/stdc++.h>
using namespace std;
int n,in1,in2,tren,prethodni,duz,koren=0;
vector<vector<int> > drvo;
stack<pair<int,int> > dfs;
pair<int,int> vrh;
vector<int> listovi;
int main()
{
ios::sync_with_stdio(false);
cin>>n;
drvo.resize(n+1);
for(int i=1;i<n;i++)
{
cin>>in1>>in2;
drvo[in1].push_back(in2);
drvo[in2].push_back(in1);
}
if(n<=20)
{
if(n==1)
{
cout<<1<<endl<<1<<" "<<1;
}
if(n==2)
{
cout<<1<<endl<<1<<" "<<2;
return 0;
}
for(int i=1;i<=n;i++)
{
if(drvo[i].size()>1)
{
dfs.push(make_pair(i,0));
break;
}
}
while(!dfs.empty())
{
vrh=dfs.top();
dfs.pop();
tren=vrh.first;
prethodni=vrh.second;
duz=drvo[tren].size();
if(duz==1)
{
listovi.push_back(tren);
}
else
{
for(int i=0;i<duz;i++)
{
if(drvo[tren][i]!=prethodni)
{
dfs.push(make_pair(drvo[tren][i],tren));
}
}
}
}
duz=listovi.size();
cout<<(duz+1)/2<<endl;
for(int i=0;i<(duz+1)/2;i++)
{
if(i==duz-i-1)
{
cout<<listovi[0]<<" "<<listovi[i]<<endl;
}
else
{
cout<<listovi[i]<<" "<<listovi[i+(duz+1)/2]<<endl;
}
}
return 0;
}
for(int i=1;i<=n;i++)
{
if(drvo[i].size()>1)
{
dfs.push(make_pair(i,0));
break;
}
}
while(!dfs.empty())
{
vrh=dfs.top();
dfs.pop();
tren=vrh.first;
prethodni=vrh.second;
duz=drvo[tren].size();
if(duz==1)
{
listovi.push_back(tren);
}
else
{
for(int i=0;i<duz;i++)
{
if(drvo[tren][i]!=prethodni)
{
dfs.push(make_pair(drvo[tren][i],tren));
}
}
}
}
duz=listovi.size();
cout<<(duz+1)/2<<endl;
for(int i=0;i<(duz+1)/2;i++)
{
if(i==duz-i-1)
{
cout<<listovi[0]<<" "<<listovi[i]<<endl;
}
else
{
cout<<listovi[i]<<" "<<listovi[(duz+1)/2+i]<<endl;
}
}
}
