#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define str string
#define pb push_back
#define pf push_front
#define in insert
#define all(v) v.begin(),v.end()
const int sz=500001,INF=1000000000;
using namespace std;
ll n,num1,num2,maxDist1=-INF,maxDist2=-INF,node1,node2;
vector<ll>dist1(sz),dist2(sz),nodes;
vector<bool>isSeen1(sz),isSeen2(sz);
vector<pair<ll,ll>>ans;
vector<vector<ll>>nums;
void dfs1(ll num)
{
isSeen1[num]=true;
for(int i=0;i<nums[num].size();i++)
{
if(!isSeen1[nums[num][i]])
{
dist1[nums[num][i]]=dist1[num]+1;
if(dist1[nums[num][i]]>maxDist1)
{
node1=nums[num][i],maxDist1=dist1[nums[num][i]];
}
dfs1(nums[num][i]);
}
}
}
void dfs2(ll num)
{
isSeen2[num]=true;
for(int i=0;i<nums[num].size();i++)
{
if(!isSeen2[nums[num][i]])
{
dist2[nums[num][i]]=dist2[num]+1;
if(dist2[nums[num][i]]>maxDist2)
{
node2=nums[num][i],maxDist2=dist2[nums[num][i]];
}
dfs2(nums[num][i]);
}
}
}
void solve()
{
cin>>n;
nums.resize(n+1);
for(int i=0;i<n-1;i++)
{
cin>>num1>>num2;
nums[num1].pb(num2),nums[num2].pb(num1);
}
dfs1(1),dfs2(node1);
ans.pb({min(node1,node2),max(node1,node2)});
for(int i=1;i<=n;i++)
{
if(nums[i].size()==1)
{
if(i!=node1 && i!=node2)
{
nodes.pb(i);
}
}
}
for(int i=0;i<nodes.size()-1;i++)
{
ans.pb({min(nodes[i],nodes[i+1]),max(nodes[i],nodes[i+1])});
}
cout<<ans.size()<<"\n";
for(int i=0;i<ans.size();i++)
{
cout<<ans[i].first<<" "<<ans[i].second<<"\n";
}
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr),cout.tie(nullptr);
ll t=1;
//cin>>t;
while(t--)
{
solve();
}
}
