#include <bits/stdc++.h>
using namespace std;
void tarjan(int u, int p, vector<vector<int>> &adj, vector<int> &tin, vector<int> &low, vector<bool> &vis, int &timer, bool &is_bridge) {
vis[u] = true;
tin[u] = low[u] = timer++;
for (auto &v : adj[u]) {
if (v == p) continue;
if (vis[v]) {
low[u] = min(low[u], tin[v]);
continue;
}
tarjan(v, u, adj, tin, low, vis, timer, is_bridge);
low[u] = min(low[u], low[v]);
is_bridge = is_bridge || (low[v] > tin[u]);
}
}
bool has_bridge(vector<vector<bool>> &adjmat) {
int n = adjmat.size();
vector<vector<int>> adj(n, vector<int>());
for (int i = 0; i < n; i++) {
for (int j = i+1; j < n; j++) {
if (adjmat[i][j] != 1) continue;
adj[i].push_back(j);
adj[j].push_back(i);
}
}
vector<int> tin(n, -1); vector<int> low(n, -1); vector<bool> vis(n, false); int timer = 0; bool is_bridge = false;
tarjan(0, 0, adj, tin, low, vis, timer, is_bridge);
return is_bridge;
}
const int MAXLOG = 18;
int n, timer = 0;
vector<vector<int>> adj, up; vector<pair<int, int>> edges;
vector<int> sz, leaves, tin, tout, depth;
void get_subtree_size(int u, int p) {
sz[u] = 1;
for (auto &v : adj[u]) {
if (v == p) continue;
get_subtree_size(v, u);
sz[u] += sz[v];
}
if (sz[u] == 1 && p != -1) leaves.push_back(u);
}
int find_centroid(int u, int p) {
for (auto &v : adj[u]) {
if (v == p) continue;
if (sz[v] * 2 > n) return find_centroid(v, u);
}
return u;
}
void dfs(int u, int p) {
tin[u] = timer++;
if (u == 0) depth[u] = 0;
else depth[u] = depth[p] + 1;
up[u][0] = p;
for (int i = 1; i < MAXLOG; i++) {
up[u][i] = up[up[u][i - 1]][i-1];
}
for (auto &v : adj[u]) {
if (v == p) continue;
dfs(v, u);
}
tout[u] = timer;
}
bool is_ancestor(int u, int v) {
return tin[u] <= tin[v] && tout[u] >= tout[v];
}
int find_lca(int u, int v) {
if (is_ancestor(u, v)) return u;
if (is_ancestor(v, u)) return v;
for (int i = MAXLOG - 1; i >= 0; i--) {
if (!is_ancestor(up[u][i], v)) {
u = up[u][i];
}
}
return up[u][0];
}
int dist(int u, int v) {
return depth[u] + depth[v] - 2 * depth[find_lca(u, v)];
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
adj.assign(n, vector<int>()); sz.assign(n, -1);
tin.assign(n, 0); tout.assign(n, 0); depth.assign(n, 0);
up.assign(n, vector<int>(MAXLOG, 0));
for (int i = 0; i < n - 1; i++) {
int u, v; cin >> u >> v;
u--; v--;
adj[u].push_back(v);
adj[v].push_back(u);
edges.push_back({u, v});
}
get_subtree_size(0, -1);
int centre = find_centroid(0, -1);
leaves.clear(); get_subtree_size(centre, -1);
dfs(centre, centre);
vector<pair<int, int>> new_edges;
set<int> lset(leaves.begin(), leaves.end());
while (!lset.empty()) {
int curnode = *lset.begin();
lset.erase(curnode);
int endnode = -1, end_dist = -1;
for (auto &v : lset) {
int vd = dist(curnode, v);
if (vd > end_dist) endnode = v, end_dist = vd;
}
lset.erase(endnode);
new_edges.push_back({curnode+1, endnode+1});
if (lset.size() == 1) {
int nnc = *lset.begin();
lset.erase(nnc);
new_edges.push_back({nnc+1, endnode+1});
}
}
cout << new_edges.size() << "\n";
for (auto &el : new_edges) {
cout << el.first << " " << el.second << "\n";
}
return 0;
}
