#include <bits/stdc++.h>
#define Loop(x,l,r) for (ll x = (l); x < (r); ++x)
#define LoopR(x,l,r) for (ll x = (r)-1; x >= (l); --x)
typedef long long ll;
typedef std::pair<int, int> pii;
typedef std::pair<ll , ll > pll;
using namespace std;
const int N = 500'010;
vector<int> A[N];
int rt = 0;
int cnt[N];
int n;
int dfs1(int v, int p)
{
cnt[v] = A[v].size() == 1;
for (int u : A[v]) {
if (u != p)
cnt[v] += dfs1(u, v);
}
return cnt[v];
}
int dfs2(int v, int p, int tot)
{
for (int u : A[v]) {
if (u == p)
continue;
if (cnt[u] > tot - cnt[u])
return dfs2(u, v, tot);
}
return v;
}
vector<int> leaves[N];
void dfs3(int v, int p, vector<int> &vec)
{
if (A[v].size() == 1)
vec.push_back(v);
for (int u : A[v])
if (u != p)
dfs3(u, v, vec);
}
bool cmp(vector<int> *a, vector<int> *b) {
return a->size() > b->size();
}
multiset<vector<int>*, bool (*)(vector<int>*, vector<int>*)> s(cmp);
int main()
{
cin.tie(0) -> sync_with_stdio(false);
cin >> n;
Loop (i,1,n) {
int v, u;
cin >> v >> u;
--v; --u;
A[v].push_back(u);
A[u].push_back(v);
}
while (A[rt].size() == 1) ++rt;
dfs1(rt, -1);
rt = dfs2(rt, -1, cnt[rt]);
for (int v : A[rt]) {
dfs3(v, rt, leaves[v]);
s.insert(&leaves[v]);
}
vector<pii> ans;
for (;;) {
auto v1 = *s.begin();
s.erase(s.begin());
if (v1->size() == 0)
break;
auto v2 = *s.begin();
s.erase(s.begin());
if (v2->size() == 0) {
ans.push_back({rt, v1->front()});
break;
}
ans.push_back({v1->back(), v2->back()});
v1->pop_back();
v2->pop_back();
s.insert(s.begin(), v1);
s.insert(s.begin(), v2);
}
cout << ans.size() << '\n';
for (auto [v, u] : ans)
cout << v+1 << ' ' << u+1 << '\n';
}
