#pragma region Macros

#include <bits/extc++.h>
using namespace std;
using namespace __gnu_pbds;
// using namespace __gnu_cxx;

#define TO_STRING(var) # var
#define pb emplace_back
#define int ll
#define endl '\n'

using ll = long long;
using ld = long double;
const ld PI = acos(-1);
const ld EPS = 1e-10;
const int INF = 1 << 30;
const ll INFL = 1LL << 61;
// const int MOD = 998244353;
const int MOD = 1000000007;

int ceil(int x, int y) {return (x > 0 ? (x + y - 1) / y : x / y);}
int POW(int x, int y) {
    if (y != (int)(y) or y < 0 or x != 0 && x != 1 && y > 64) {cout << "Error" << endl;return 0;}
    if (y == 0) return 1;
    if (y % 2 == 0) return POW(x * x, y / 2);
    return x * POW(x, y - 1);
}

__attribute__((constructor))
void constructor() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);
}

#pragma endregion

signed main() {
    int N;
    cin >> N;

    vector<int> A(N);
    unordered_map<int,int> mp;
    for (int i = 0; i < N; i++) {
        cin >> A[i];
        mp[A[i]]++;
    }
    sort(A.begin(), A.end());

    if (N == 1) {cout << 0 << endl; return 0;}

    int mex;
    bool flag = false;
    for (int i = 0; i <= 1e5; i++) {
        if (mp[i] >= 3) flag = true;
        if (mp[i] == 0) {
            mex = i;
            break;
        } else if (mp[i] == 1) {
            if (i == *max_element(A.begin(),A.end()) && flag == false) {
                mex = i;
                break;
            }
            i++;
            while (i != 1e5 + 1) {
                if (flag && mp[i] <= 1) {
                    mex = i;
                    break;
                } else if (mp[i] <= 2) {
                    mex = i;
                    break;
                }
                i++;
            }
            if (i == 1e5 + 1) mex = i;
            break;
        }
    }
    cout << mex << endl;
}