#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vb = vector<bool>;
using vvb = vector<vb>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vc = vector<char>;
using vvc = vector<vc>;
using vs = vector<string>;
const ll mod = 1e9 + 7,inf = 1e18;
#define pb push_back
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
void setIO()
{
fast;
}
int n = 5e5 + 5,t;
vvi adj(n);
vi in(n),out(n),leaves;
void dfs(int u,int p)
{
in[u] = ++t;
for (int v:adj[u])if (v != p)
dfs(v,u);
out[u] = ++t;
if (adj[u].size() == 1)leaves.pb(u);//leaves includes the root
}
bool cmp(int a,int b)
{
return in[a] < in[b];
}
int main()
{
setIO();
cin>>n;
for (int i = 0;i<n - 1;i++){int u,v;cin>>u>>v;adj[u].pb(v),adj[v].pb(u);}
dfs(1,-1);
sort(leaves.begin(),leaves.end(),cmp);
int ans = (int)leaves.size();
ans = (ans%2 == 1 ? ans/2 + 1 : ans/2);
cout<<ans<<'\n';
for (int i = 0;i<(int)leaves.size()/2;i++)
cout<<leaves[i]<<" "<<leaves[(i + ans)%(int)leaves.size()]<<'\n';
if ((int)leaves.size()%2 == 1)
cout<<leaves[(int)leaves.size()/2]<<" "<<leaves[0];
return 0;
}
