#include<bits/stdc++.h>
using namespace std;
//#pragma GCC optimize("Ofast")

#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

int main(){
    ll n;cin >> n;
    vl v(100010);
    rep(i,n){
        ll a;cin >> a;v[a]++;
    }
    ll mx=0;
    rep(i,100010){
        if(v[i]==0){
            mx=i;break;
        }
    }
    ll over=0;
    rep(i,100010){
        if(v[i]>2){
            over+=v[i]-2;v[i]=2;
        }
        if(i>mx){
            over+=v[i];v[i]=0;
        }
    }
    rep(i,100010){
        if(v[i]==0){
            cout << i << endl;return 0;
        }
        else if(v[i]==1){
            if(over>0){
                v[i]++;over=-1;continue;
            }
            else if(over==0){
                if(mx-1==i){
                    cout << i << endl;return 0;
                }
                else{
                    v[mx-1]--;over=-1;continue;
                }
            }
            cout << i << endl;return 0;
        }
    }
}