#include <bits/stdc++.h>
using namespace std;
auto start = std::chrono::system_clock::now();
inline void skj() {
  std::chrono::time_point<std::chrono::system_clock> end;
  end = std::chrono::system_clock::now();
  std::chrono::duration<double> elapsed_seconds = end - start;
  std::time_t end_time = std::chrono::system_clock::to_time_t(end);
  cerr << "Time taken " << elapsed_seconds.count() * 1000 << "\n";
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
inline long long binexp(long long a, long long b, long long m) {
  if (a == 0) {
    return 0;
  }
  long long res = 1;
  a %= m;
  while (b) {
    if (b & 1) res = (res * 1ll * a) % m;
    a = (a * 1ll * a) % m;
    b >>= 1;
  }
  return res;
}
inline void solve_me_senpai() {
  long long n;
  cin >> n;
  vector<long long> arr(n - 1);
  set<long long> avail;
  for (long long i = 1; i <= n; i++) {
    avail.insert(i);
  }
  for (long long i = 0; i < n - 1; i++) {
    cin >> arr[i];
  }
  vector<pair<long long, long long> > edges;
  set<long long> nodes;
  nodes.insert(arr[0]);
  avail.erase(arr[0]);
  for (long long i = 0; i < n - 1; i++) {
    if (i == n - 2 || (nodes.find(arr[i + 1]) != nodes.end())) {
      edges.push_back({arr[i], *(--avail.end())});
      avail.erase(--avail.end());
      nodes.insert(edges.back().second);
    } else {
      edges.push_back({arr[i], arr[i + 1]});
      nodes.insert(arr[i + 1]);
      if ((avail.find(arr[i + 1]) != avail.end())) {
        avail.erase(arr[i + 1]);
      }
    }
  }
  cout << arr[0];
  cout << "\n";
  for (auto i : edges) {
    cout << i.first << " " << i.second;
    cout << "\n";
  }
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  long long a = 1;
  while (t--) {
    solve_me_senpai();
    a++;
  }
  return 0;
}
