#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using vi = vector<int>;
template<typename T1,typename T2> bool chmax(T1 &a, const T2 b) {if (a < b) {a = b; return true;} else return false; }
template<typename T1,typename T2> bool chmin(T1 &a, const T2 b) {if (a > b) {a = b; return true;} else return false; }

signed main() {

    int n;cin>>n;
    map<int,int> mp;
    rep(i,n){
        int a;cin>>a;
        mp[a]++;
    }
    set<int> st;
    for(auto &e:mp){
        e.second/=2;
        if(e.second!=0)st.insert(e.first);
    }
    int ans = 0;
    while(st.count(ans))ans++;
    cout<<ans<<endl;
    return 0;
}