#include <bits/stdc++.h>
using namespace std;

char buf[(1<<21)+5],*p1,*p2;
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)
int read(){
	int f=0,x=0;
	char ch=getchar();
	while(!isdigit(ch)){f|=(ch=='-');ch=getchar();}
	while(isdigit(ch)){x=x*10+ch-'0';ch=getchar();}
	return f?-x:x;
}

const int N=5e6+5;

int a[N], lst[N][22], dp[N], links[N], nex[500005][22][22], now[22][22];
pair <int,int> val[N];

bool cmp(int x,int y)
{
	return dp[x]<dp[y];
}

int main()
{
	int n=read();
	for(int i=1;i<=n;i++)
		a[i]=read()-1;

	for(int i=1;i<=n;i++)
	{
		for(int j=0;j<21;j++)
			lst[i][j]=lst[i-1][j];
		lst[i][a[i]]=i;
	}

	for(int i=1;i<n;i++)
		val[i]=make_pair(min(a[i],a[i+1]),max(a[i],a[i+1]));

	memset(now,0x3f,sizeof(now));
	for(int i=n-1;i>=1;i--)
	{
		links[i]=now[val[i].first][val[i].second];
		now[val[i].first][val[i].second]=i;
		if((i-1)%10==0)
		{
			for(int c1=0;c1<21;c1++)
				for(int c2=c1;c2<21;c2++)
					nex[(i-1)/10][c1][c2]=now[c1][c2];
		}
	}

	memset(dp,0xcf,sizeof(dp));
	dp[0]=0;
	int ans=0x3f3f3f3f;
	for(int S=0;S<1<<21;S++)
	{
		if(dp[S]<0 || ans<=__builtin_popcount(S))
			continue;

		if(dp[S]>=n-1)
			memset(now,0x3f,sizeof(now));
		else
		{
			memcpy(now,nex[dp[S]/10],sizeof(now));
			for(int i=dp[S]/10*10+1;i<=dp[S];i++)
				now[val[i].first][val[i].second]=links[i];
		}

		int pos=0x3f3f3f3f;
		for(int i=0;i<21;i++)
			if(!((S>>i)&1))
				for(int j=i;j<21;j++)
					if(!((S>>j)&1))
						pos=min(pos,now[i][j]+1);

		if(pos==0x3f3f3f3f)
			ans=min(ans,__builtin_popcount(S));
		else
		{
			int del=0;
			for(int i=0;i<21;i++)
				if(!((S>>i)&1))
					dp[S+(1<<i)-del]=max(dp[S+(1<<i)-del],lst[pos][i]);
				else
					del=1<<i;
		}
	}
	printf("%d\n",ans);
	return 0;
}