#include<bits/stdc++.h> //:3
using namespace std;
typedef long long ll;
#define all(a) (a).begin(), (a).end()
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define pi pair<int, int>
#define sz(x) (int)((x).size())
#define int long long
const int dx[] = {0, 1, 0, -1};
const int dy[] = {1, 0, -1, 0};
const ll inf = 2e9;
const ll mod = 1e9 + 7;
const int N = 2e6 + 11;
const ll INF64 = 3e18 + 1;
const double eps = 1e-14;
const double PI = acos(-1);
//ifstream in(".in");
//ofstream out(".out");
int n, m;
vector<int>v[N];
vector<int>leaves;
void dfs(int nod, int p){
if(sz(v[nod]) == 1)leaves.push_back(nod);
for(int it: v[nod]){
if(it == p)continue;
dfs(it, nod);
}
}
void solve(){
cin >> n;
for(int i = 1, x, y; i < n; i++){
cin >> x >> y;
v[x].push_back(y);
v[y].push_back(x);
}
dfs(1, 0);
//cout << "L" << '\n';
//for(auto it : leaves)cout << it << ' ';cout << '\n';
int ans = (sz(leaves) + 1)/2;
cout << ans << '\n';
for(int i = 0; i < ans; i ++){
cout << leaves[i] << ' ' << leaves[i + sz(leaves)/2] << '\n';
}
}
int32_t main(){
ios_base :: sync_with_stdio(0); cin.tie(0); cout.tie(0);
int T = 1;
//cin >> T;
while(T--){
solve();
}
}
