#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define IOS ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
const ll inf=1e9+10;
const ll maxn=5e5+100;
int n;
vector<int>v[maxn],leaf;
int x[maxn],y[maxn];
vector<pair<int,int>>ans;
void dfs(int nod,int par){
if(v[nod].size()==1)leaf.pb(nod);
for(auto ch:v[nod]){
if(ch==par)continue;
dfs(ch,nod);
}
}
int d[2],dist[maxn][2];
pair<int,int>getfar(int nod,int par,int dep){
pair<int,int>ret={dep,nod};
for(auto ch:v[nod]){
if(ch==par)continue;
ret=max(ret,getfar(ch,nod,dep+1));
}
return ret;
}
inline void diam(){
auto el=getfar(1,0,0);
d[0]=el.second;
el=getfar(d[0],0,0);
d[1]=el.second;
}
void dfs(int nod,int par,int dep,bool k){
dist[nod][k]=dep;
for(auto x:v[nod]){
if(x==par)continue;
dfs(x,nod,dep+1,k);
}
}
bool com0(int x,int y){
return dist[x][0]>dist[y][0];
}
bool com1(int x,int y){
return dist[x][1]>dist[y][1];
}
bool vis[maxn];
int main()
{
IOS
cin>>n;
for(int i=0;i<n-1;i++){
int a,b;
cin>>a>>b;
v[a].pb(b);
v[b].pb(a);
}
diam();
//cout<<d[0]<<' '<<d[1]<<endl;
dfs(d[0],0,0,0);
dfs(d[1],0,0,1);
for(int i=0;i<n;i++){
x[i]=i+1;
y[i]=i+1;
}
sort(x,x+n,com0);
sort(y,y+n,com1);
//for(int i=0;i<n;i++)cout<<x[i]<<' ';cout<<endl;
//for(int i=0;i<n;i++)cout<<y[i]<<' ';cout<<endl;
int cur0=0,cur1=0;
while(cur0<n&&cur1<n){
if(vis[x[cur0]]||v[x[cur0]].size()>1){cur0++;continue;}
if(vis[y[cur1]]||v[y[cur1]].size()>1){cur1++;continue;}
if(x[cur0]==y[cur1]){cur0++;continue;}
ans.pb({x[cur0],y[cur1]});
vis[x[cur0]]=1;
vis[y[cur1]]=1;
cur0++;
cur1++;
}
//cout<<cur0<<' '<<cur1<<endl;
int l0=cur0,r0=n-1;
while(l0<r0){
if(v[x[l0]].size()>1||vis[x[l0]]){l0++;continue;}
if(v[x[r0]].size()>1||vis[x[r0]]){r0--;continue;}
ans.pb({x[l0],x[r0]});
vis[x[l0]]=1;
vis[x[r0]]=1;
l0++;
r0--;
}
if(l0==r0&&v[x[r0]].size()==1&&!vis[x[r0]]){
if(x[r0]==d[0])ans.pb({x[r0],d[1]});
else ans.pb({x[r0],d[0]});
}
int l1=cur1,r1=n-1;
while(l1<r1){
if(v[y[l1]].size()>1||vis[y[l1]]){l1++;continue;}
if(v[y[r1]].size()>1||vis[y[r1]]){r1--;continue;}
ans.pb({y[l1],y[r1]});
vis[y[l1]]=1;
vis[y[r1]]=1;
l1++;
r1--;
}
if(l1==r1&&v[y[r1]].size()==1&&!vis[y[r1]]){
if(y[r1]==d[0])ans.pb({y[r1],d[1]});
else ans.pb({y[r1],d[0]});
}
cout<<ans.size()<<endl;
for(auto it:ans){
cout<<it.first<<' '<<it.second<<endl;
}
return 0;
}
