#include <bits/stdc++.h>
#define fi first
#define se second
#define FOR(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)
#define FORLL(i, a, b) for (ll i = a, _b = b; i <= _b; ++i)
#define FORDLL(i, a, b) for (ll i = a, _b = b; i >= _b; --i)
#define all(x) x.begin(), x.end()
#define uni(x) sort(all(x)), x.erase(unique(all(x)), x.end())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define dbg(...) debug(#__VA_ARGS__, __VA_ARGS__)
template<typename T>
void __prine_one(const char *&s, const T &x)
{
while (*s == ' ') ++s;
const char *p = s;
int bal = 0;
while (*s)
{
if (*s == '(') ++bal;
else if (*s == ')') --bal;
else if (*s == ',' && bal == 0) break;
++s;
}
cerr.write(p, s - p) << " = " << x;
if (*s == ',')
{
cerr << " , ";
++s;
}
}
template<typename... Args>
void debug(const char *s, Args... args)
{
cerr << "[ ";
int dummy[] = {0, (__prine_one(s, args), 0)...};
(void)dummy;
cerr << " ]\n\n";
}
template<class X>
bool maximize(X &a, const X &b)
{
if (b > a)
{
a = b;
return true;
}
return false;
}
template<class X>
bool minimize(X &a, const X &b)
{
if (b < a)
{
a = b;
return true;
}
return false;
}
// --------------------------------------------------------------------------------------------
const int maxn = 5e5 + 3;
int n, tour[maxn << 1][20], tin[maxn], tdfs, h[maxn], lg[maxn << 1], rTin[maxn], rTdfs;
vector<int> g[maxn];
// --------------------------------------------------------------------------------------------
#define MIN_H(x, y) (h[x] < h[y] ? x : y)
int lca(int u, int v)
{
u = tin[u]; v = tin[v];
if (u > v) swap(u, v);
int k = lg[v - u + 1];
return MIN_H(tour[u][k], tour[v - (1 << k) + 1][k]);
}
int get_dist(int u, int v)
{
return h[u] + h[v] - 2 * h[lca(u, v)];
}
struct Node
{
int u, v;
Node() {}
Node(int u, int v) : u(u), v(v) {}
bool operator > (const Node &other) const
{
return get_dist(u, v) > get_dist(other.u, other.v);
}
};
Node seg[maxn << 3];
Node combine(Node a, Node b)
{
Node c;
if (a.u == 0) return b;
if (b.u == 0) return a;
if (a.v == 0 && b.v == 0)
{
c.u = a.u;
c.v = b.u;
return c;
}
if (a.v == 0) swap(a, b);
c = a;
maximize(c, Node(a.u, b.u));
maximize(c, Node(a.v, b.u));
if (b.v)
{
maximize(c, Node(a.u, b.v));
maximize(c, Node(a.v, b.v));
maximize(c, b);
}
return c;
}
void upd(int id, int l, int r, int pos, int val)
{
if (l == r)
{
seg[id].u = val;
seg[id].v = 0;
return;
}
int mid = l + r >> 1;
if (pos <= mid) upd(id << 1, l, mid, pos, val);
else upd(id << 1 | 1, mid + 1, r, pos, val);
seg[id] = combine(seg[id << 1], seg[id << 1 | 1]);
}
void dfs_pre(int u, int p)
{
tin[u] = ++tdfs;
rTin[u] = ++rTdfs;
tour[tdfs][0] = u;
for (int v : g[u])
{
if (v == p) continue;
h[v] = h[u] + 1;
dfs_pre(v, u);
tour[++tdfs][0] = u;
}
}
void solve()
{
cin >> n;
FOR(i, 2, n)
{
int u, v; cin >> u >> v;
if (u > v) swap(u, v);
g[u].push_back(v);
g[v].push_back(u);
}
dfs_pre(1, -1);
FOR(j, 1, 19)
FOR(i, 1, tdfs - (1 << j) + 1)
tour[i][j] = MIN_H(tour[i][j - 1], tour[i + (1 << j - 1)][j - 1]);
FOR(i, 2, tdfs)
lg[i] = lg[i >> 1] + 1;
FOR(i, 1, n)
if (g[i].size() == 1)
upd(1, 1, n, rTin[i], i);
bool improve = true;
vector<pii> vec;
int lst = 0;
while (improve)
{
improve = false;
Node cur = seg[1];
if (cur.u && cur.v)
{
lst = cur.u;
vec.push_back({cur.u, cur.v});
improve = true;
upd(1, 1, n, rTin[cur.u], 0);
upd(1, 1, n, rTin[cur.v], 0);
}
else if (cur.u)
{
vec.push_back({lst, cur.u});
}
}
cout << int(vec.size()) << '\n';
for (const auto &p : vec)
cout << p.fi << ' ' << p.se << '\n';
}
signed main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define TASK "TEST"
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
freopen(TASK".OUT", "w", stdout);
}
solve();
return 0;
}
