#include <bits/stdc++.h>
using namespace std;
#define Task ""
struct __Init__ {
__Init__() {
cin.tie(nullptr)->sync_with_stdio(false);
if (fopen(Task".inp", "r")) {
freopen(Task".inp", "r", stdin);
freopen(Task".out", "w", stdout); }
}
} __init__;
using ll = long long;
#ifdef LOCAL
#define debug(x) cerr << "[" #x " = " << x << "]\n";
#else
#define debug(...)
#endif // LOCAL
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define fi first
#define se second
#define For(i, l, r) for (int i = (l); i <= (r); ++i)
#define Ford(i, r, l) for (int i = (r); i >= (l); --i)
#define Rep(i, n) For (i, 0, (n) - 1)
#define Repd(i, n) Ford (i, (n) - 1, 0)
template<class C> int isz(const C& c) { return c.size(); }
template<class T> bool chmin(T& a, const T& b) { if (a > b) { a = b; return true; } return false; }
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } return false; }
constexpr int eps = 1e-9;
constexpr int inf = 1e9;
constexpr ll linf = 1e18;
// =============================================================================
constexpr int maxN = 5e5 + 5;
int n;
vector<int> g[maxN];
vector<pair<int, int>> ans;
int d[maxN];
int trace[maxN];
queue<int> q;
vector<int> child;
void Bfs(int s) {
For (i, 1, n) {
d[i] = -1;
trace[i] = 0;
}
d[s] = 0;
trace[s] = -1;
q.push(s);
while (isz(q)) {
int u = q.front(); q.pop();
for (int v : g[u]) {
if (d[v] == -1) {
d[v] = d[u] + 1;
trace[v] = u;
q.push(v);
}
}
}
}
void Dfs(int u, int p) {
bool hasChild = false;
for (int v : g[u]) {
if (v != p) {
hasChild = true;
Dfs(v, u);
}
}
if (!hasChild) {
child.push_back(u);
}
}
signed main() {
cin >> n;
For (i, 1, n - 1) {
int u, v; cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
ans.reserve(n);
Bfs(1);
int s = max_element(d + 1, d + n + 1) - d;
Bfs(s);
int t = max_element(d + 1, d + n + 1) - d;
ans.emplace_back(s, t);
child.reserve(n);
int pre = -1;
for (int u = t; u != -1; u = trace[u]) {
for (int v : g[u]) {
if (v == pre || v == trace[u]) {
continue;
}
Dfs(v, u);
}
pre = u;
}
Rep (i, isz(child) - 1) {
ans.emplace_back(child[i], child[i + 1]);
}
cout << isz(ans) << '\n';
for (const auto& p : ans) {
cout << p.fi << ' ' << p.se << '\n';
}
}
/*
*/
