#include <iostream>
#include <algorithm>
#include <iterator>
#include <vector>
#include <numeric>

using namespace std;

int main() {
    int n;
    cin>>n;
    vector <int> v(n);
    vector <int> nums(1000);
    iota (nums.begin (), nums.end (), 0);
    copy_n (istream_iterator <int> (cin), n, v.begin());
    auto ans = remove_if (nums.begin (), nums.end (), [&] (int x) {return count (v.begin(), v.end(), x) != 0;});
    cout << nums[0];
    return 0;
}