#include<bits/stdc++.h>
using namespace std;
namespace file_read{
    namespace input_file_io{
        char ib[1<<25],*ip1=ib,*ip2=ib;
        inline char gc(){
#ifndef ONLINE_JUDGE
			return getchar();
#else
            return (ip1==ip2&&(ip2=(ip1=ib)+fread(ib,1,1<<24,stdin)),ip1==ip2?EOF:*ip1++);
#endif
        }
        inline int read(){
            int x=0;
            char c=gc();
            while(c<'0'||c>'9')c=gc();
            while(c>='0'&&c<='9'){
                x=(x<<3)+(x<<1)+(c^'0');
                c=gc();
            }
            return x;
        }
    };
    namespace output_file_io{
        char ob[1<<25],*op=ob;
        inline void pc(char c){
            *op++=c;
        }
        void write(int x){
            if(x<0){
                pc('-');
                x=-x;
            }
            if(x==0)pc('0');
            static int number_stack[40];
            int total_count=0;
            while(x)number_stack[++total_count]=x%10,x/=10;
            while(total_count){
                pc(number_stack[total_count]+'0');
                --total_count;
            }
        }
        inline void final_write(){
            fwrite(ob,op-ob,1,stdout);
    	}
    };
	using namespace input_file_io;
	using namespace output_file_io;
};
using namespace file_read;
int a[5000005];
int f[1<<21];
int z1[5000005][22],z2[5000005][22];
int main(){
	int n=read(),k=0;
	for(int i=1;i<=n;++i)a[i]=read(),k=max(k,a[i]);
	for(int i=n;i>=1;--i){
		memcpy(z1[i],z1[i+1],88);
		int x=z1[i][a[i]];z1[i][a[i]]=i;
		if(x)memcpy(z2[i],z2[x],88);
		if(i<n)z2[i][a[i+1]]=i;
	}
	memset(f,-63,sizeof(f));
	f[0]=0;
	int ans=1e9;
	for(int S=0;S<(1<<k);++S)if(f[S]>=0){
		int bp=__builtin_popcount(S);
		if(bp>=ans)continue;
		if(f[S]<n){
			int x=f[S],mn=n+1;
			for(int i=1;i<=k;++i)if(z1[x+1][i]&&!(S>>(i-1)&1)){
				int y=z1[x+1][i];
				for(int j=1;j<=k;++j)if(z2[y][j]&&!(S>>(j-1)&1)){
					mn=min(mn,z2[y][j]);
				}
			}
			f[S]=mn-1;
		}
		if(f[S]>=n){
			ans=min(ans,bp);
			continue;
		}
		if(f[S]+1<=n){
			int t=a[f[S]+1];
			int SS=S|(1<<(t-1));
			int S2=SS&((1<<(t-1))-1);
			if(S2)SS^=1<<(31-__builtin_clz(S2));
			f[SS]=max(f[SS],f[S]);
		}
		if(f[S]+2<=n){
			int t=a[f[S]+2];
			int SS=S|(1<<(t-1));
			int S2=SS&((1<<(t-1))-1);
			if(S2)SS^=1<<(31-__builtin_clz(S2));
			f[SS]=max(f[SS],f[S]);
		}
	}
	printf("%d\n",ans);
	return 0;
}