#include <iostream>
using namespace std;

int main()
{
    int n;
    cin >> n;
    int d[100005]{0};
    for (int i = 0; i < n; i++)
    {
        int a;
        cin >> a;
        d[a]++;
    }
    int s = 0;
    bool more2 = false;
    bool one = false;
    for (int a = 0; a <= n + 1; a++)
    {
        s += d[a];
        if (d[a] == 0 || (d[a] == 1 && one))
        {
            if (s == n && d[a] == 0 && one && !more2)
            {
                cout << a - 1 << endl;
            }
            else
            {
                cout << a << endl;
            }
            break;
        }
        if (d[a] == 1)
        {
            one = true;
        }
        if (d[a] > 2)
        {
            more2 = true;
        }
    }
}