#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll,ll>;
#define fix(x) fixed << setprecision(x)
#define asc(x) x, vector<x>, greater<x>
#define rep(i, n) for(ll i = 0; i < n; i++)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<int> cnt(100100, 0);
    rep(i,n){
        int a;
        cin >> a;
        cnt[a]++;
    }
    int i = 0, rem = 0;
    for(;;i++){
        while(cnt[i]<1 && rem){
            rem--;
            cnt[i]++;
        }
        while(cnt[i]>3){
            cnt[i] -= 2;
            rem++;
        }
        if(cnt[i]<2) break;
    }
    cout << i << '\n';
    return 0;
}