#include <bits/stdc++.h>
using namespace std;
int main () {
    int N;
    cin >> N;
    vector<int> C(100010, 0);
    for (int i = 0; i < N; i ++) {
        int a;
        cin >> a;
        C[a] ++;
    }
    int fl = 0;
    int sum = 0;
    bool alt = true;
    while (C[fl] >= 2) {
        sum += C[fl];
        alt = alt && (C[fl] == 2);
        fl ++;
    }
    if (C[fl] == 0 || (sum + C[fl] == N && alt)) {
        cout << fl << endl;
        return 0;
    }
    fl ++;
    while (C[fl] >= 2) {
        fl ++;
    }
    cout << fl << endl;
}