#include<bits/stdc++.h>
using namespace std;
const int MAXN = 500010;
vector<int> v[MAXN];
vector<int> fol;
void solve(int x, int p){
bool folha = (v[x].size() == 1);
for(auto y : v[x]) if(y != p){
solve(y, x);
}
if(folha) fol.push_back(x);
}
int main(){
int n; cin >> n;
for(int i = 0; i + 1 < n; ++i){
int a, b; cin >> a >> b;
a--; b--;
v[a].push_back(b);
v[b].push_back(a);
}
solve(0, -1);
int m = fol.size();
int mid = (m + 1) / 2;
cout << mid << "\n";
for(int i = 0; i < mid; ++i){
cout << fol[i] + 1 << " " << fol[min(i + mid, m - 1)] + 1 << "\n";
}
}
