#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

int N;
int C[202020];


void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N;
	FOR(i,N) {
		cin>>x;
		C[x]++;
	}
	x=-1;
	y=-1;
	FOR(i,101010) {
		if(C[i]>2) {
			x=i;
			C[i]--;
			break;
		}
		if(C[i]==1) {
			if(y>=0) {
				x=i;
				C[i]--;
				break;
			}
			y=i;
		}
	}
	if(y>=0&&x==-1) {
		FOR(i,101010) if(C[i]==2) x=i;
		C[y]++;
		C[x]--;
		x=-1;
	}
	
	FOR(i,101010) {
		if(C[i]<2&&x>=0) {
			C[i]++;
			x=-1;
		}
		if(C[i]<2) {
			cout<<i<<endl;
			return;
		}
	}
		
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}