#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
  }
  vector<int> cnt(N + 2, 0);
  for (int i = 0; i < N; i++){
    if (A[i] < N + 2){
      cnt[A[i]]++;
    }
  }
  vector<pair<int, int>> P;
  for (int i = 0; i < N + 2; i++){
    if (cnt[i] < 2){
      P.push_back(make_pair(i, cnt[i]));
    }
  }
  if (P[0].second == 0){
    cout << P[0].first << endl;
  } else if (P[1].second == 0){
    if (N == P[1].first * 2 - 1){
      cout << P[1].first - 1 << endl;
    } else {
      cout << P[1].first << endl;
    }
  } else {
    cout << P[1].first << endl;
  }
}