#include <bits/stdc++.h> #include <atcoder/all> using namespace std; using namespace atcoder; typedef long long ll; #define rep(i, n) for (int i = 0; i < (int)(n); i++) #define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--) #define repk(i, k, n) for (int i = k; i < (int)(n); i++) #define all(v) v.begin(), v.end() #define mod1 1000000007 #define mod2 998244353 #define mod3 100000007 #define vi vector<int> #define vs vector<string> #define vc vector<char> #define vl vector<ll> #define vvi vector<vector<int>> #define vvc vector<vector<char>> #define vvl vector<vector<ll>> #define vvvi vector<vector<vector<int>>> #define vvvl vector<vector<vector<ll>>> #define pii pair<int,int> #define pil pair<int,ll> #define pli pair<ll, int> #define pll pair<ll, ll> #define vpii vector<pair<int,int>> #define vpll vector<pair<ll,ll>> #define vvpii vector<vector<pair<int,int>>> #define vvpll vector<vector<pair<ll,ll>>> template<typename T> void debug(T e){ cerr << e << endl; } template<typename T> void debug(vector<T> &v){ rep(i, v.size()){ cerr << v[i] << " "; } cerr << endl; } template<typename T> void debug(vector<vector<T>> &v){ rep(i, v.size()){ rep(j, v[i].size()){ cerr << v[i][j] << " "; } cerr << endl; } } template<typename T> void debug(vector<pair<T, T>> &v){ rep(i,v.size()){ cerr << v[i].first << " " << v[i].second << endl; } } template<typename T> void debug(set<T> &st){ for (auto itr = st.begin(); itr != st.end(); itr++){ cerr << *itr << " "; } cerr << endl; } template<typename T> void debug(multiset<T> &ms){ for (auto itr = ms.begin(); itr != ms.end(); itr++){ cerr << *itr << " "; } cerr << endl; } template<typename T> void debug(map<T, T> &mp){ for (auto itr = mp.begin(); itr != mp.end(); itr++){ cerr << itr->first << " " << itr->second << endl; } } void debug_out(){ cerr << endl; } template <typename Head, typename... Tail> void debug_out(Head H, Tail... T){ cerr << H << " "; debug_out(T...); } int main(){ int N; cin >> N; vector<int> A(N); rep(i,N){ cin >> A[i]; } sort(all(A)); map<int, int> mp; for (int i = 0; i < N; i++){ mp[A[i]]++; } int dmex = 0; int nokori = 0; for (int i = 0; i < N; i++){ if (mp[i] >= 2){ dmex = i + 1; nokori += (mp[i] - 2); } else{ break; } } if (mp[dmex] == 1 && N > 2 * dmex + 1){ dmex++; while(mp[dmex] >= 2 && N > 2 * dmex + 1){ dmex++; } } cout << dmex << endl; }