#include <bits/stdc++.h>
// #include "roads.h"
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define ALL(v) (v).begin(), (v).end()
#define MASK(i) (1LL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
// mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng(1);
ll rngesus(ll l, ll r){return ((ull) rng()) % (r - l + 1) + l;}
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll LASTBIT(ll mask){return mask & (-mask);}
ll pop_cnt(ll mask){return __builtin_popcountll(mask);}
ll ctz(ll mask){return __builtin_ctzll(mask);}
ll clz(ll mask){return __builtin_clzll(mask);}
ll logOf(ll mask){return 63 - clz(mask);}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b){a = b; return true;}
return false;
}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b){a = b; return true;}
return false;
}
template <class T>
void printArr(T a, string separator = " ", string finish = "\n", ostream& out = cout){
for(auto i: a) out << i << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
const int N = 5e5 + 69;
int n;
vector<int> graph[N];
vector<int> ans;
void dfs(int u, int p){
if (graph[u].size() == 1) ans.push_back(u);
for(int v: graph[u]) if (v != p){
dfs(v, u);
}
}
int main(void){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n;
for(int i = 1; i<n; ++i) {
int u, v; cin >> u >> v;
graph[u].push_back(v);
graph[v].push_back(u);
}
int root = 0;
for(int i = 1; i<=n; ++i) if (graph[i].size() > 1) root = i;
dfs(root, root);
int res = (ans.size() + 1) / 2;
cout << res << "\n";
for(int i = 0; i<res; ++i) cout << ans[i] << " " << ans[i + ans.size() / 2] << "\n";
return 0;
}
