#include <bits/stdc++.h>
using namespace std;
#define SZ(s) (int)s.size()
vector <vector <int>> v;
vector <pair<int,int>> ans;
vector <int> v1;
int n, a, b, cn;
void dfs(int x, int y = -1){
for(auto i : v[x]){
if(i == y) continue;
dfs(i,x);
}
if(SZ(v[x]) == 1) v1.push_back(x);
return;
}
int main(){
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
v.resize(n+1);
for(int i = 1; i < n; i++){
int u1, u2;
cin >> u1 >> u2;
v[u1].push_back(u2);
v[u2].push_back(u1);
}
dfs(1);
cout << (SZ(v1)+1)/2 << '\n';
int x = (SZ(v1))/2;
for(int i = 0; i < (SZ(v1)+1)/2; i++){
cout << v1[i] << ' ' << v1[i+x] << '\n';
}
// if(SZ(v1) % 2 == 1) cout << v1.back() << ' ' << v1[SZ(v1)-2] << '\n';
}
