#include <set>
#include <cstdio>
using namespace std;
set<int>s,t;
int a[200010],f[200010],g[200010];
bool b[200010];
int n,Ans;
int main(){
	freopen("cardgame.in","r",stdin);
	freopen("cardgame.out","w",stdout);
	scanf("%d",&n);
	for(int i=1;i<=n;i++)scanf("%d",&a[i]),b[a[i]]=true;
	for(int i=1;i<=n<<1;i++)if(!b[i])s.insert(i),t.insert(-i);
	for(int i=1;i<=n;i++){
		set<int>::iterator it=s.lower_bound(a[i]);
		if(it!=s.end())s.erase(it),f[i]=f[i-1]+1;
		else f[i]=f[i-1];
	}
	for(int i=n;i;i--){
		set<int>::iterator it=t.lower_bound(-a[i]);
		if(it!=t.end())t.erase(it),g[i]=g[i+1]+1;
		else g[i]=g[i+1];
	}
	for(int i=0;i<=n;i++)if(Ans<f[i]+g[i+1])Ans=f[i]+g[i+1];
	printf("%d\n",Ans);
}