#include<bits/stdc++.h>
using namespace std;
#define rc(x) return cout<<x<<endl,0
#define pb push_back
#define dbg(x) cout << #x << '=' << x << '\n';
#define all(a) (a).begin(), (a).end()
#define ll long long
#define sz size()
#define x first
#define y second
#define pi pair <int, int>
#define pii pair <int, pi>
#define vi vector <int>
const ll mod = 1e9 + 7;
int n, deg[500001];
vector <int> g[500001];
vector <int> leaves;
void dfs(int cur, int p){
//if(deg[cur]==1) cout<<p<<" "<<cur<<" ";
if(deg[cur]==1) leaves.pb(cur);
for(int i: g[cur]){
if(i!=p) dfs(i, cur);
}
}
int32_t main(){
ios_base :: sync_with_stdio(0); cin.tie(); cout.tie();
cin>>n;
for(int i=1; i<=n-1; i++){
int x, y;
cin>>x>>y;
g[x].pb(y);
g[y].pb(x);
deg[x]++, deg[y]++;
}
dfs(1, 0);
int xd=(leaves.size()+1)/2;
cout<<xd<<"\n";
sort(all(leaves));
for(int i=0; i<xd; i++) cout<<leaves[i]<<" "<<leaves[i+leaves.size()/2]<<"\n";
}
