#include <bits/stdc++.h>
#define ll long long
using namespace std;
int main() {
ios::sync_with_stdio(0), cin.tie(0);
int n; cin >> n;
vector <int> adj[n+1];
for (int i = 1, u, v; i < n; i++) cin >> u >> v, adj[u].emplace_back(v), adj[v].emplace_back(u);
auto bfs = [&] (int st) {
queue <int> q;
int vis[n+1]; memset(vis, 0, sizeof(vis));
int rc = -1;
q.emplace(st);
while (!q.empty()) {
int u = q.front(); q.pop();
rc = u;
for (auto v : adj[u]) if (!vis[v]) q.emplace(v), vis[v] = 1;
}
return rc;
};
int D1 = bfs(1);
int D2 = bfs(D1);
int dia[n+1]; memset(dia, 0, sizeof(dia));
function <int(int, int)> fd = [&] (int u, int p) {
dia[u] = (u == D1 || u == D2);
for (auto v : adj[u]) if (v != p) dia[u] = max(dia[u], fd(v, u));
return dia[u];
};
fd(D1, D1);
vector <vector <int>> V;
function <void(int, int, vector <int> &)> dfs = [&] (int u, int p, vector <int> &k) {
for (auto v : adj[u]) if (v != p && !dia[v]) k.emplace_back(v), dfs(v, u, k);
};
for (int i = 1; i <= n; i++) if (dia[i]) {
vector <int> k;
dfs(i, i, k);
if (k.size()) V.emplace_back(k);
}
vector <pair <int, int>> ans;
int ss = 0, sz = V.size();
for (auto &e : V) {
if (e.size() >= 2) {
int u = e.back(); e.pop_back();
int v = e.back(); e.pop_back();
ss = 1;
ans.emplace_back(u, D1);
ans.emplace_back(v, D2);
if (e.empty()) e.swap(V[--sz]);
break;
}
}
while (sz > 1) {
ans.emplace_back(V[0].back(), V[1].back());
V[0].pop_back();
V[1].pop_back();
cerr << sz << '\n';
if (V[0].empty()) V[0].swap(V[--sz]);
if (V[1].empty()) V[1].swap(V[--sz]);
}
while (!V[0].empty()) ans.emplace_back(V[0].back(), D1), V[0].pop_back();
if (!ss) ans.emplace_back(D1, D2);
cout << ans.size() << '\n';
for (auto &[x, y] : ans) cout << x << ' ' << y << '\n';
}
