#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  VI cnt(100010);
  rep(_, n) {
    int x;
    cin >> x;
    cnt[x]++;
  }
  int c1 = 0;
  int rest = n;
  for(int i = 0;; i++) {
    if (cnt[i] == 0) {
      cout << i << '\n';
      break;
    } else if (cnt[i] == 1) {
      rest--;
      if (rest == 0) {
        cout << i << '\n';
        break;
      }
      c1++;
      rest--;
      if (c1 == 2) {
        cout << i << '\n';
        break;
      }
    } else {
      rest -= 2;
    }
  }
}