#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef double db;
typedef string str;
typedef pair<int,int> pi;
typedef pair<ll,ll> pl;
typedef pair<db,db> pd;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<db> vd;
typedef vector<str> vs;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<pd> vpd;
#define mp make_pair
#define f first
#define s second
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) (x).rbegin(), (x).rend()
#define rsz resize
#define ins insert
#define ft front()
#define bk back()
#define pf push_front
#define pb push_back
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)
const int MOD = 1e9+7; // 998244353;
const int MX = 2e5+5;
const ll INF = 1e18;
const ld PI = acos((ld)-1);
const int xd[4] = {1,0,-1,0}, yd[4] = {0,1,0,-1};
template<class T> bool ckmin(T& a, const T& b) {
return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) {
return a < b ? a = b, 1 : 0; }
int pct(int x) { return __builtin_popcount(x); }
int bit(int x) { return 31-__builtin_clz(x); } // floor(log2(x))
int cdiv(int a, int b) { return a/b+!(a<0||a%b == 0); } // division of a by b rounded up, assumes b > 0
// INPUT
template<class A> void re(complex<A>& c);
template<class A, class B> void re(pair<A,B>& p);
template<class A> void re(vector<A>& v);
template<class A, size_t SZ> void re(array<A,SZ>& a);
template<class T> void re(T& x) { cin >> x; }
void re(db& d) { str t; re(t); d = stod(t); }
void re(ld& d) { str t; re(t); d = stold(t); }
template<class H, class... T> void re(H& h, T&... t) { re(h); re(t...); }
template<class A> void re(complex<A>& c) { A a,b; re(a,b); c = {a,b}; }
template<class A, class B> void re(pair<A,B>& p) { re(p.f,p.s); }
template<class A> void re(vector<A>& x) { trav(a,x) re(a); }
template<class A, size_t SZ> void re(array<A,SZ>& x) { trav(a,x) re(a); }
// TO_STRING
#define ts to_string
template<class A, class B> str ts(pair<A,B> p);
template<class A> str ts(complex<A> c) { return ts(mp(c.real(),c.imag())); }
str ts(bool b) { return b ? "true" : "false"; }
str ts(char c) { str s = ""; s += c; return s; }
str ts(str s) { return s; }
str ts(const char* s) { return (str)s; }
str ts(vector<bool> v) {
bool fst = 1; str res = "{";
F0R(i,sz(v)) {
if (!fst) res += ", ";
fst = 0; res += ts(v[i]);
}
res += "}"; return res;
}
template<size_t SZ> str ts(bitset<SZ> b) {
str res = ""; F0R(i,SZ) res += char('0'+b[i]);
return res; }
template<class T> str ts(T v) {
bool fst = 1; str res = "{";
for (const auto& x: v) {
if (!fst) res += ", ";
fst = 0; res += ts(x);
}
res += "}"; return res;
}
template<class A, class B> str ts(pair<A,B> p) {
return "("+ts(p.f)+", "+ts(p.s)+")"; }
// OUTPUT
template<class A> void pr(A x) { cout << ts(x); }
template<class H, class... T> void pr(const H& h, const T&... t) {
pr(h); pr(t...); }
void ps() { pr("\n"); } // print w/ spaces
template<class H, class... T> void ps(const H& h, const T&... t) {
pr(h); if (sizeof...(t)) pr(" "); ps(t...); }
// DEBUG
void DBG() { cerr << "]" << endl; }
template<class H, class... T> void DBG(H h, T... t) {
cerr << to_string(h); if (sizeof...(t)) cerr << ", ";
DBG(t...); }
#ifdef LOCAL // compile with -DLOCAL
#define dbg(...) cerr << "[" << #__VA_ARGS__ << "]: [", DBG(__VA_ARGS__)
#else
#define dbg(...) 42
#endif
// FILE I/O
void setIn(string s) { freopen(s.c_str(),"r",stdin); }
void setOut(string s) { freopen(s.c_str(),"w",stdout); }
void unsyncIO() { ios_base::sync_with_stdio(0); cin.tie(0); }
void setIO(string s = "") {
unsyncIO();
// cin.exceptions(cin.failbit);
// throws exception when do smth illegal
// ex. try to read letter into int
if (sz(s)) { setIn(s+".in"), setOut(s+".out"); } // for USACO
}
mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());
const int mx = 500005;
vi adj[mx];
int sub[mx];
int to;
int cent;
int tot;
void genSub(int node, int prv = -1){
if(sz(adj[node]) == 1){
sub[node] = 1;
tot++;
return;
}
for(auto u: adj[node]){
if(u == prv) continue;
genSub(u, node);
sub[node]+=sub[u];
}
}
void genCent(int node, int prv = -1){
for(auto u: adj[node]){
if(u == prv) continue;
if(sub[u]*2 > tot){
genCent(u, node);
return;
}
}
cent = node;
}
vi leaves[mx];
void labelLeaves(int node, int prv, int label){
if(sz(adj[node]) == 1) leaves[label].pb(node);
for(auto u: adj[node]){
if(u == prv) continue;
labelLeaves(u, node, label);
}
}
vi szs[mx];
int main() {
setIO();
int n;
cin >> n;
for(int i = 0; i < n-1; i++){
int a, b;
cin >> a >> b;
adj[a].pb(b);
adj[b].pb(a);
}
for(int i = 1; i <= n; i++){
if(sz(adj[i]) > 1){
to = i;
}
}
genSub(to);
genCent(to);
for(int i = 0; i < sz(adj[cent]); i++){
labelLeaves(adj[cent][i], cent, i);
}
int m = sz(adj[cent]);
vpi ans;
int curmx = 0;
for(int i = 0; i < m; i++){
szs[sz(leaves[i])].pb(i);
ckmax(curmx, sz(leaves[i]));
}
int mx1 = curmx;
int mx2 = curmx;
/*dbg(cent);
for(int i = 0; i < m; i++){
dbg(i, szs[i]);
}*/
for(int i = 0; i < (tot+1)/2; i++){
while(mx1 > 0 && sz(szs[mx1]) == 0){
mx1--;
}
while(mx2 > 0 && ((mx2 > mx1) || (sz(szs[mx2]) == 0) || ((mx2 == mx1) && (sz(szs[mx1]) == 1)))){
mx2--;
}
//dbg(ans, mx1, mx2);
if(mx2 != 0){
int a = szs[mx1].bk;
szs[mx1].pop_back();
int b = szs[mx2].bk;
szs[mx2].pop_back();
szs[mx1-1].pb(a);
szs[mx2-1].pb(b);
ans.pb(mp(leaves[a].bk, leaves[b].bk));
leaves[a].pop_back();
leaves[b].pop_back();
}
else if(mx1 > 0){
int a = szs[mx1].bk;
int node1 = leaves[a].bk;
ans.pb(mp(node1, cent));
}
}
ps(sz(ans));
for(auto u: ans){
cout << u.f << " " << u.s << "\n";
}
// you should actually read the stuff at the bottom
}
/* stuff you should look for
* int overflow, array bounds
* special cases (n=1?)
* do smth instead of nothing and stay organized
* WRITE STUFF DOWN
*/
