// हर हर महादेव
using namespace std;
#include <bits/stdc++.h>
#define ll long long int
#define ld long double
#define uid(a, b) uniform_int_distribution<int>(a, b)(rng)
mt19937 rng((unsigned int) chrono::steady_clock::now().time_since_epoch().count());
#ifdef shivang_ka_laptop
	#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__);
	#define booga cerr << "booga" << endl;
#else
	#define debug(...) 42;
	#define booga 9;
#endif

template <typename T> std::ostream& operator<<(std::ostream& stream, const vector<T>& vec){ for(size_t i = 0; i < vec.size(); i++){stream << vec[i];if(i != vec.size() - 1)stream << ' ';}; return stream; }
template <typename T> std::istream& operator>>(std::istream& stream, vector<T>& vec) { for(T &x : vec)stream >> x;return stream; }
template <typename T,typename U> std::ostream& operator<<(std::ostream& stream, const pair<T,U>& pr){ stream << pr.first << ' ' << pr.second; return stream; }
template <typename T,typename U> std::istream& operator>>(std::istream& stream, pair<T,U>& pr){ stream >> pr.first >> pr.second; return stream; }
template <typename T, typename U> void operator+=(vector<T>& vec, const U value) { for(T &x : vec)x += value; }
template <typename T, typename U> void operator-=(vector<T>& vec, const U value) { for(T &x : vec)x -= value; }
template <typename T, typename U> void operator*=(vector<T>& vec, const U value) { for(T &x : vec)x *= value; }
template <typename T, typename U> void operator/=(vector<T>& vec, const U value) { for(T &x : vec)x /= value; }
template <typename T> void operator++(vector<T>& vec) { vec += 1; }
template <typename T> void operator++(vector<T>& vec,int) { vec += 1; }
template <typename T> void operator--(vector<T>& vec) { vec -= 1; }
template <typename T> void operator--(vector<T>& vec,int) { vec -= 1; }
template <typename T,typename U, typename V> void operator+=(pair<T,U>& vec, const V value) { vec.first += value;vec.second += value; }
template <typename T,typename U, typename V> void operator-=(pair<T,U>& vec, const V value) { vec.first -= value;vec.second -= value; }
template <typename T,typename U, typename V> void operator*=(pair<T,U>& vec, const V value) { vec.first *= value;vec.second *= value; }
template <typename T,typename U, typename V> void operator/=(pair<T,U>& vec, const V value) { vec.first /= value;vec.second /= value; }
template <typename T,typename U> void operator++(pair<T,U>& vec) { vec += 1; } 
template <typename T,typename U> void operator++(pair<T,U>& vec,int) { vec += 1; } 
template <typename T,typename U> void operator--(pair<T,U>& vec) { vec -= 1; } 
template <typename T,typename U> void operator--(pair<T,U>& vec,int) { vec -= 1; } 

template <typename A, typename B>string to_string(pair<A, B> p);
template <typename A, typename B, typename C>string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>string to_string(tuple<A, B, C, D> p);
string to_string(const string& s) { return '"' + s + '"'; }
string to_string(char c) {string s;s += c;return s; }
string to_string(const char* s) {return to_string((string) s); }
string to_string(bool b) {return (b ? "1" : "0"); }
string to_string(vector<bool> v) {bool first = true;string res = "{";for (int i = 0; i < static_cast<int>(v.size()); i++) {if (!first) {res += ", ";}first = false;res += to_string(v[i]);}res += "}";return res;}
template <size_t N>string to_string(bitset<N> v) {string res = "";for (size_t i = 0; i < N; i++) {res += static_cast<char>('0' + v[i]);}return res;}
template <typename A>string to_string(A v) {bool first = true;string res = "{";for (const auto &x : v) {if (!first) {res += ", ";}first = false;res += to_string(x);}res += "}";return res;}
template <typename A, typename B>string to_string(pair<A, B> p) {return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";}
template <typename A, typename B, typename C>string to_string(tuple<A, B, C> p) {return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";}
template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";}
template<class T> bool ckmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }

void debug_out() { cerr << endl; } template <typename Head, typename... Tail>void debug_out(Head H, Tail... T) {cerr << " " << to_string(H);debug_out(T...);}
void bharo(int N_N) { return; }template <typename Head, typename... Tail>void bharo(int N_N, Head &H, Tail & ... T) {H.resize(N_N);bharo(N_N,T...);}
void safai() { return; }template <typename Head, typename... Tail>void safai(Head &H, Tail & ... T) {H.clear();safai(T...);}


void testcase(){
	int n;
	cin >> n;
	vector<int> a(n);
	map<int,int> freq;
	for(int i = 0; i < n; i++){
		cin >> a[i];
		freq[a[i]]++;
	}
	int gon = 0;
	bool hua = false;
	for(int i = 0; i <= n+5; i++){
		if(freq[i] >= 2){
			gon += 2;
			continue;
		}
		if(hua || freq[i] == 0){
			cout << i;
			return;
		}
		hua = true;
		gon++;
		int av = n - gon;
		if(av < 1){
			cout << i;
			return;
		}
		gon++;
	}
}

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	int tt = 1;
	//cin >> tt;
	while(tt--){
		testcase();
	}
	return (0-0);
}