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#include <bits/stdc++.h>

using namespace std;
using ll = long long;
#define int ll
using PII = pair<int, int>;
template <typename T> using V = vector<T>;
template <typename T> using VV = vector<V<T>>;
template <typename T> using VVV = vector<VV<T>>;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define PB push_back

const ll INF = (1LL<<60);
const int MOD = 1000000007;

template <typename T> T &chmin(T &a, const T &b) { return a = min(a, b); }
template <typename T> T &chmax(T &a, const T &b) { return a = max(a, b); }
template <typename T> bool IN(T a, T b, T x) { return a<=x&&x<b; }
template<typename T> T ceil(T a, T b) { return a/b + !!(a%b); }
template<class S,class T>
ostream &operator <<(ostream& out,const pair<S,T>& a){
  out<<'('<<a.first<<','<<a.second<<')';
  return out;
}
template<class T>
ostream &operator <<(ostream& out,const vector<T>& a){
  out<<'[';
  REP(i, a.size()) {out<<a[i];if(i!=a.size()-1)out<<',';}
  out<<']';
  return out;
}

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

signed main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);

  int n;
  cin >> n;
  VV<int> g(n);
  REP(i, n-1) {
    int u, v; cin >> u >> v; u--, v--;
    g[u].PB(v);
    g[v].PB(u);
  }

  if(n == 2) {
    cout << "1" << endl;
    cout << "1 2" << endl;
    return 0;
  }

  // 葉の数と根を決める
  int m = 0, root = -1;
  REP(i, n) {
    if(g[i].size() == 1) m++;
    else root = i;
  }

  // 重心を求める
  auto findCentroid = [&]() {
    V<int> size(n), centroid;
    function<void(int,int)> dfs = [&](int ver, int par) {
      bool is_centroid = true;
      for(auto to: g[ver]) {
        if(to == par) continue;
        dfs(to, ver);
        if(size[to] > m/2) is_centroid = false;
        size[ver] += size[to];
      }
      if(g[ver].size() == 1) size[ver]++;
      if(m - size[ver] > m/2) is_centroid = false;
      if(is_centroid) centroid.PB(ver);
    };
    dfs(root, -1);
    return centroid;
  };
  auto centoroid = findCentroid();

  // 重心からdfsして葉の集合を求める
  int idx = 0;
  VV<int> leaf;
  function<void(int,int)> dfs = [&](int ver, int par) {
    if(g[ver].size()==1) leaf[idx].PB(ver);
    for(auto to: g[ver]) {
      if(to == par) continue;
      dfs(to, ver);
    }
  };
  for(auto i: g[centoroid[0]]) {
    leaf.PB(V<int>());
    dfs(i, centoroid[0]);
    idx++;
  }

  // leaf[i].size() の上位2つを取って経路を一つつくるを繰り返す
  V<PII> ans;
  priority_queue<PII> que;
  REP(i, leaf.size()) {
    if(leaf[i].size()) {
      que.push({leaf[i].size(), i});
    }
  }
  while(que.size()) {
    if(que.size() == 1) {
      PII p1 = que.top(); que.pop();
      ans.PB({leaf[p1.second].back(), centoroid[0]});
      continue;
    }
    PII p1 = que.top(); que.pop();
    PII p2 = que.top(); que.pop();
    ans.PB({leaf[p1.second].back(), leaf[p2.second].back()});
    leaf[p1.second].pop_back();
    leaf[p2.second].pop_back();
    if(leaf[p1.second].size()) que.push({leaf[p1.second].size(), p1.second});
    if(leaf[p2.second].size()) que.push({leaf[p2.second].size(), p2.second});
  }

  cout << ans.size() << endl;
  for(auto i: ans) cout << i.first+1 << " " << i.second+1 << endl;

  return 0;
}