#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define fi first
#define se second
#define __builtin_popcount __builtin_popcountll
#define all(x) (x).begin(), (x).end()
#define BIT(x, i) (((x) >> (i)) & 1)
#define MASK(x) ((1ll << (x)))
#define debug(a, l, r) for (int _i = (l); _i <= (r); ++_i) cout << (a)[_i] << ' '; cout << '\n';
const int MAXN = 5e5 + 5;
int n, deg[MAXN], st[MAXN], timer = 0;
vector<int> adj[MAXN];
vector<int> leaves;
void dfs(int u, int par = -1) {
st[u] = ++timer;
for (int v : adj[u]) {
if (v == par) continue;
dfs(v, u);
}
}
void solve() {
cin >> n;
for (int i = 1; i < n; ++i) {
int u, v; cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
deg[u]++; deg[v]++;
}
dfs(1);
for (int u = 1; u <= n; ++u) if (deg[u] == 1) leaves.push_back(u);
sort(all(leaves), [&] (int x, int y) {
return st[x] < st[y];
});
int numLeaves = (int)leaves.size();
cout << (numLeaves + 1) / 2 << '\n';
for (int i = 0; i < numLeaves / 2; ++i) {
cout << leaves[i] << ' ' << leaves[i + numLeaves / 2] << '\n';
}
if (numLeaves & 1) cout << leaves[numLeaves - 1] << ' ' << leaves[0];
}
signed main() {
#ifdef NCTHANH
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
solve();
return 0;
}
