#include<bits/stdc++.h>
using namespace std;
struct freader {
    FILE *f;
#   ifdef ONLINE_JUDGE
    char buf[1048577], *p1, *p2;
#       define fgetc(f) (p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1048576, f), p1 == p2) ? EOF : *p1++)
#   endif
#   ifdef BOOLTRANS
    bool neof;
#       define NEOF(c) ((c) != EOF || (neof = 0))
#   else
#       define NEOF(c) ((c) != EOF)
#   endif
    freader(FILE *_f = stdin) : f(_f) {
#       ifdef BOOLTRANS
        neof = 1;
#       endif
#       ifdef ONLINE_JUDGE
        setvbuf(f, NULL, _IONBF, 0);
        p1 = p2 = buf;
#       endif
    }
    void read(char &x) {
        for (x = fgetc(f); NEOF(x) && x <= ' '; x = fgetc(f));
        return;
    }
    void read(char *s) {
        for (*s = fgetc(f); NEOF(*s) && *s <= ' '; *s = fgetc(f));
        for (s++; NEOF(*s = fgetc(f)) && *s > ' '; s++);
        *s = '\0';
        return;
    }
    void read(float x) {return fputs("Error: Unable to read float.", stderr), void();}
    void read(double x) {return fputs("Error: Unable to read double.", stderr), void();}
    void read(long double x) {return fputs("Error: Unable to read long double.", stderr), void();}
    template<typename T> void read(T &x) {
        char c(fgetc(f));
#       ifdef NEGATIVE
        for (; NEOF(c) && (c < '0' || c > '9') && c != '-'; c = fgetc(f));
        if (c == '-')
            for (c = fgetc(f), x = 0; NEOF(c) && c >= '0' && c <= '9'; c = fgetc(f)) x = (x << 3) + (x << 1) - (c ^ '0');
        else
            for (x = 0; NEOF(c) && c >= '0' && c <= '9'; c = fgetc(f)) x = (x << 3) + (x << 1) + (c ^ '0');
#       else
        for (; NEOF(c) && (c < '0' || c > '9'); c = fgetc(f));
        for (x = 0; NEOF(c) && c >= '0' && c <= '9'; c = fgetc(f)) x = (x << 3) + (x << 1) + (c ^ '0');
#       endif
        return;
    }
#   if __cplusplus >= 201103
    template<typename T, typename...Args> void read(T &x, Args &...args) {return read(x), read(args...);}
#   endif
    template<typename T> freader &operator >> (T &x) {
#       ifdef BOOLTRANS
        return *this ? read(x), *this : *this;
#       else
        return read(x), *this;
#       endif
    }
#   ifdef BOOLTRANS
    operator bool() {return neof;}
#   endif
#   ifdef ONLINE_JUDGE
#       undef fgetc
#   endif
#   undef NEOF
} fin;
int n,a[5000005],lim;
int dp[1<<21|5];
int nxt[250005][21][21],tmp[25],tt,pp[21][21];
int go(int sta,int i){
	for(int s=a[i];s>=0;s--){
		if((sta>>s)&1){
			sta^=(1<<s);
			break;
		}
	}
	sta|=(1<<a[i]);
	return sta;
}
int B=20;
int bel[5000005];
int main(){
	fin.read(n);
	for(int i=1;i<=n;i++)fin.read(a[i]),a[i]--;
	for(int i=n;i;i--){
		lim=max(lim,a[i]);
	}
	for(int i=1;i<=n;i++)bel[i]=(i-1)/B+1;
	for(int i=0;i<=lim;i++)for(int j=0;j<=lim;j++)pp[i][j]=nxt[bel[n-1]+1][i][j]=n+1;
	for(int i=n-1;i>=0;i--){
		if(i)pp[a[i]][a[i+1]]=i;
		if(bel[i]!=bel[i-1]){
			for(int j=0;j<=lim;j++)for(int k=0;k<=lim;k++)nxt[bel[i]][j][k]=pp[j][k];
		}
	}
	int mx=1<<(lim+1);
	for(int sta=0;sta<mx;sta++){
		tt=0;
		if(!dp[sta]&&sta)continue;
		if(dp[sta]>=n-1)continue;
		for(int j=0;j<=lim;j++){
			if((sta>>j)&1);
			else tmp[++tt]=j;
		}
		int rp=n+1,u=dp[sta];
		for(int i=u+1;bel[u]==bel[i];i++){
			if((sta>>a[i])&1);
			else{
				if((sta>>a[i+1])&1);
				else rp=min(rp,i);
			}
		}
		for(int i=1;i<=tt;i++){
			for(int j=1;j<=tt;j++){
				rp=min(rp,nxt[bel[u]+1][tmp[i]][tmp[j]]);
			}
		}
		if(rp==1)rp--;
		else if((sta>>a[rp-1])&1)rp--;
		else rp-=2;
		dp[sta]=rp;
		if(rp+2<=n){
			int to=go(sta,rp+2);
			dp[to]=max(dp[to],rp+2);
		}
		if(rp+1<=n){
			int to=go(sta,rp+1);
			dp[to]=max(dp[to],rp+1);
		}
	}
	int ans=1e9;
	for(int i=0;i<mx;i++){
		if(dp[i]>=n-1){
			int popc=0;
			for(int j=0;j<=lim;j++){
				if((i>>j)&1)popc++;
			}
			ans=min(ans,popc);
		}
	}
	cout<<ans;
	return 0;
}