#include<bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint=modint998244353; //1000000007;
using ll=long long;
using pp=pair<int,int>;
#define sr string 
#define vc vector
#define db double
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define pb push_back
#define all(v) v.begin(),v.end()
#define pque priority_queue
#define bpc(a) __builtin_popcount(a)
int main(){
	int n;cin>>n;
	vc<int>v(2e5,0);
	rep(i,n){
		int a;cin>>a;
		v[a]++;
	}
	bool bo=true;
	int ans;
	for(int i=0;i<(int)2e5;i++){
		if(v[i]==0){
			ans=i;
			break;
		}
		if(bo&&v[i]==1){
			bo=false;
			continue;
		}
		if(!bo&&v[i]==1){
			ans=i;break;
		}
	}
	if(n&1)if(ans==n/2+1)ans--;
	cout<<ans;
}