#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define ll long long
#define mp(x, y) make_pair(x, y)
#define sz(v) ((int) (v).size())
#define all(v) (v).begin(), (v).end()
#define MASK(i) (1LL << (i))
#define BIT(x, y) (((x) >> (y)) & 1)
#define pb push_back
#define max_rng *max_element
#define min_rng *min_element
#define rep(i, n) for(int i = 1, _n = (n); i <= _n; ++i)
#define forn(i, a, b) for(int i = (a), _b = (b); i <= _b; ++i)
#define ford(i, a, b) for(int i = (a), _b = (b); i >= _b; --i)
template <class X, class Y>
inline bool maximize(X &x, Y y) {
return (x < y ? x = y, true : false);
}
template <class X, class Y>
inline bool minimize(X &x, Y y) {
return (x > y ? x = y, true : false);
}
template <class X>
inline void compress(vector<X> &a) {
sort(all(a));
a.resize(unique(all(a)) - a.begin());
}
int ctz(ll x) { return __builtin_ctzll(x); }
int lg(ll x) { return 63 - __builtin_clzll(x); }
int popcount(ll x) { return __builtin_popcount(x); }
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rand(ll l, ll r) {
return l + abs((ll) rng()) % (r - l + 1);
}
const ll oo = (ll) 1e17;
const int inf = (ll) 1e9 + 7, mod = (ll) 1e9 + 7;
const int N = 5e5 + 5, M = 250005, BASE = 307, LOG = 23;
void add(int &x, int y) { x += y; if (x >= mod) x -= mod; }
void sub(int &x, int y) { x -= y; if (x < 0) x += mod; }
int n;
vector<int> g[N];
vector<int> node;
void dfs(int u, int p = 0) {
if (sz(g[u]) == 1) node.pb(u);
for (int v : g[u]) if (v != p) {
dfs(v, u);
}
return;
}
int main(void) {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n;
rep(i, n - 1) {
int u, v;
cin >> u >> v;
g[u].pb(v);
g[v].pb(u);
}
rep(i, n) if (sz(g[i]) > 1) {
dfs(i);
break;
}
int m = sz(node);
cout << (m + 1) / 2 << '\n';
forn(i, 0, m / 2 - 1) cout << node[i] << ' ' << node[i + m / 2] << '\n';
if (m & 1) cout << node[0] << ' ' << node.back();
return 0;
}
