/*  _              _     _                                             _       __      __      __   
   / \     _   _  | |_  | |__     ___    _ __   _    ___   _ __ ___   | | __  / /_    / /_    / /_  
  / _ \   | | | | | __| | '_ \   / _ \  | '__| (_)  / __| | '_ ` _ \  | |/ / | '_ \  | '_ \  | '_ \ 
 / ___ \  | |_| | | |_  | | | | | (_) | | |     _  | (__  | | | | | | |   <  | (_) | | (_) | | (_) |
/_/   \_\  \__,_|  \__| |_| |_|  \___/  |_|    (_)  \___| |_| |_| |_| |_|\_\  \___/   \___/   \___/ 
[Created Time:       2025-02-02 20:50:43]
[Last Modified Time: 2025-02-02 21:42:16] */
#ifdef LOCAL
#include<bits/stdc++.h>
#include"debug.h"
#else
#pragma GCC optimize("Ofast", "unroll-loops")
#include<bits/stdc++.h>
#define D(...) ((void)0)
#endif
using namespace std; using ll = long long;
#define For(i, j, k) for ( int i = (j) ; i <= (k) ; i++ )
#define Fol(i, j, k) for ( int i = (j) ; i >= (k) ; i-- )
namespace FastIO
{
#define USE_FastIO
// ------------------------------
// #define DISABLE_MMAP
// ------------------------------
#if ( defined(LOCAL) || defined(_WIN32) ) && !defined(DISABLE_MMAP)
#define DISABLE_MMAP
#endif
#ifdef LOCAL
	inline void _chk_i() {}
	inline char _gc_nochk() { return getchar(); }
	inline char _gc() { return getchar(); }
	inline void _chk_o() {}
	inline void _pc_nochk(char c) { putchar(c); }
	inline void _pc(char c) { putchar(c); }
	template < int n > inline void _pnc_nochk(const char *c) { for ( int i = 0 ; i < n ; i++ ) putchar(c[i]); }
#else
#ifdef DISABLE_MMAP
	inline constexpr int _READ_SIZE = 1 << 18; inline static char _read_buffer[_READ_SIZE + 40], *_read_ptr = nullptr, *_read_ptr_end = nullptr; static inline bool _eof = false;
	inline void _chk_i() { if ( __builtin_expect(!_eof, true) && __builtin_expect(_read_ptr_end - _read_ptr < 40, false) ) { int sz = _read_ptr_end - _read_ptr; if ( sz ) memcpy(_read_buffer, _read_ptr, sz); char *beg = _read_buffer + sz; _read_ptr = _read_buffer, _read_ptr_end = beg + fread(beg, 1, _READ_SIZE, stdin); if ( __builtin_expect(_read_ptr_end != beg + _READ_SIZE, false) ) _eof = true, *_read_ptr_end = EOF; } }
	inline char _gc_nochk() { return __builtin_expect(_eof && _read_ptr == _read_ptr_end, false) ? EOF : *_read_ptr++; }
	inline char _gc() { _chk_i(); return _gc_nochk(); }
#else
#include<sys/mman.h>
#include<sys/stat.h>
	inline static char *_read_ptr = (char *)mmap(nullptr, [] { struct stat s; return fstat(0, &s), s.st_size; } (), 1, 2, 0, 0);
	inline void _chk_i() {}
	inline char _gc_nochk() { return *_read_ptr++; }
	inline char _gc() { return *_read_ptr++; }
#endif
	inline constexpr int _WRITE_SIZE = 1 << 18; inline static char _write_buffer[_WRITE_SIZE + 40], *_write_ptr = _write_buffer;
	inline void _chk_o() { if ( __builtin_expect(_write_ptr - _write_buffer > _WRITE_SIZE, false) ) fwrite(_write_buffer, 1, _write_ptr - _write_buffer, stdout), _write_ptr = _write_buffer; }
	inline void _pc_nochk(char c) { *_write_ptr++ = c; }
	inline void _pc(char c) { *_write_ptr++ = c, _chk_o(); }
	template < int n > inline void _pnc_nochk(const char *c) { memcpy(_write_ptr, c, n), _write_ptr += n; }
	inline struct _auto_flush { inline ~_auto_flush() { fwrite(_write_buffer, 1, _write_ptr - _write_buffer, stdout); } } _auto_flush;
#endif
#define println println_ // don't use C++23 std::println
	template < class T > inline constexpr bool _is_signed = numeric_limits < T >::is_signed;
	template < class T > inline constexpr bool _is_unsigned = numeric_limits < T >::is_integer && !_is_signed < T >;
#if __SIZEOF_LONG__ == 64
	template <> inline constexpr bool _is_signed < __int128 > = true;
	template <> inline constexpr bool _is_unsigned < __uint128_t > = true;
#endif
	inline bool _isgraph(char c) { return c >= 33; }
	inline bool _isdigit(char c) { return 48 <= c && c <= 57; } // or faster, remove c <= 57
	constexpr struct _table {
#ifndef LOCAL
	int i[65536];
#endif
	char o[40000]; constexpr _table() :
#ifndef LOCAL
	i{},
#endif
	o{} {
#ifndef LOCAL
	for ( int x = 0 ; x < 65536 ; x++ ) i[x] = -1; for ( int x = 0 ; x <= 9 ; x++ ) for ( int y = 0 ; y <= 9 ; y++ ) i[x + y * 256 + 12336] = x * 10 + y;
#endif
	for ( int x = 0 ; x < 10000 ; x++ ) for ( int y = 3, z = x ; ~y ; y-- ) o[x * 4 + y] = z % 10 + 48, z /= 10; } } _table;
	template < class T, int digit > inline constexpr T _pw10 = 10 * _pw10 < T, digit - 1 >;
	template < class T > inline constexpr T _pw10 < T, 0 > = 1;
	inline void read(char &c) { do c = _gc(); while ( !_isgraph(c) ); }
	inline void read_cstr(char *s) { char c = _gc(); while ( !_isgraph(c) ) c = _gc(); while ( _isgraph(c) ) *s++ = c, c = _gc(); *s = 0; }
	inline void read(string &s) { char c = _gc(); s.clear(); while ( !_isgraph(c) ) c = _gc(); while ( _isgraph(c) ) s.push_back(c), c = _gc(); }
	template < class T, bool neg >
#ifndef LOCAL
	__attribute__((no_sanitize("undefined")))
#endif
	inline void _read_int_suf(T &x) { _chk_i(); char c; while
#ifndef LOCAL
	( ~_table.i[*reinterpret_cast < unsigned short *& >(_read_ptr)] ) if constexpr ( neg ) x = x * 100 - _table.i[*reinterpret_cast < unsigned short *& >(_read_ptr)++]; else x = x * 100 + _table.i[*reinterpret_cast < unsigned short *& >(_read_ptr)++]; if
#endif
	( _isdigit(c = _gc_nochk()) ) if constexpr ( neg ) x = x * 10 - ( c & 15 ); else x = x * 10 + ( c & 15 ); }
	template < class T, enable_if_t < _is_signed < T >, int > = 0 > inline void read(T &x) { char c; while ( !_isdigit(c = _gc()) ) if ( c == 45 ) { _read_int_suf < T, true >(x = -( _gc_nochk() & 15 )); return; } _read_int_suf < T, false >(x = c & 15); }
	template < class T, enable_if_t < _is_unsigned < T >, int > = 0 > inline void read(T &x) { char c; while ( !_isdigit(c = _gc()) ); _read_int_suf < T, false >(x = c & 15); }
	inline void write(bool x) { _pc(x | 48); }
	inline void write(char c) { _pc(c); }
	inline void write_cstr(const char *s) { while ( *s ) _pc(*s++); }
	inline void write(const string &s) { for ( char c : s ) _pc(c); }
	template < class T, bool neg, int digit > inline void _write_int_suf(T x) { if constexpr ( digit == 4 ) _pnc_nochk < 4 >(_table.o + ( neg ? -x : x ) * 4); else _write_int_suf < T, neg, digit / 2 >(x / _pw10 < T, digit / 2 >), _write_int_suf < T, neg, digit / 2 >(x % _pw10 < T, digit / 2 >); }
	template < class T, bool neg, int digit > inline void _write_int_pre(T x) { if constexpr ( digit <= 4 ) if ( digit >= 3 && ( neg ? x <= -100 : x >= 100 ) ) if ( digit >= 4 && ( neg ? x <= -1000 : x >= 1000 ) ) _pnc_nochk < 4 >(_table.o + ( neg ? -x : x ) * 4); else _pnc_nochk < 3 >(_table.o + ( neg ? -x : x ) * 4 + 1); else if ( digit >= 2 && ( neg ? x <= -10 : x >= 10 ) ) _pnc_nochk < 2 >(_table.o + ( neg ? -x : x ) * 4 + 2); else _pc_nochk(( neg ? -x : x ) | 48); else { constexpr int cur = 1 << __lg(digit - 1); if ( neg ? x <= -_pw10 < T, cur > : x >= _pw10 < T, cur > ) _write_int_pre < T, neg, digit - cur >(x / _pw10 < T, cur >), _write_int_suf < T, neg, cur >(x % _pw10 < T, cur >); else _write_int_pre < T, neg, cur >(x); } }
	template < class T, enable_if_t < _is_signed < T >, int > = 0 > inline void write(T x) { if ( x >= 0 ) _write_int_pre < T, false, numeric_limits < T >::digits10 + 1 >(x); else _pc_nochk(45), _write_int_pre < T, true, numeric_limits < T >::digits10 + 1 >(x); _chk_o(); }
	template < class T, enable_if_t < _is_unsigned < T >, int > = 0 > inline void write(T x) { _write_int_pre < T, false, numeric_limits < T >::digits10 + 1 >(x), _chk_o(); }
	template < size_t N, class ...T > inline void _read_tuple(tuple < T... > &x) { read(get < N >(x)); if constexpr ( N + 1 != sizeof...(T) ) _read_tuple < N + 1, T... >(x); }
	template < size_t N, class ...T > inline void _write_tuple(const tuple < T... > &x) { write(get < N >(x)); if constexpr ( N + 1 != sizeof...(T) ) _pc(32), _write_tuple < N + 1, T... >(x); }
	template < class ...T > inline void read(tuple < T... > &x) { _read_tuple < 0, T... >(x); }
	template < class ...T > inline void write(const tuple < T... > &x) { _write_tuple < 0, T... >(x); }
	template < class T1, class T2 > inline void read(pair < T1, T2 > &x) { read(x.first), read(x.second); }
	template < class T1, class T2 > inline void write(const pair < T1, T2 > &x) { write(x.first), _pc(32), write(x.second); }
	template < class T > inline auto read(T &x) -> decltype(x.read(), void()) { x.read(); }
	template < class T > inline auto write(const T &x) -> decltype(x.write(), void()) { x.write(); }
	template < class T1, class ...T2 > inline void read(T1 &x, T2 &...y) { read(x), read(y...); }
	template < class ...T > inline void read_cstr(char *x, T *...y) { read_cstr(x), read_cstr(y...); }
	template < class T1, class ...T2 > inline void write(const T1 &x, const T2 &...y) { write(x), write(y...); }
	template < class ...T > inline void write_cstr(const char *x, const T *...y) { write_cstr(x), write_cstr(y...); }
	template < class T > inline void print(const T &x) { write(x); }
	inline void print_cstr(const char *x) { write_cstr(x); }
	template < class T1, class ...T2 > inline void print(const T1 &x, const T2 &...y) { write(x), _pc(32), print(y...); }
	template < class ...T > inline void print_cstr(const char *x, const T *...y) { write_cstr(x), _pc(32), print_cstr(y...); }
	inline void println() { _pc(10); }
	inline void println_cstr() { _pc(10); }
	template < class ...T > inline void println(const T &...x) { print(x...), _pc(10); }
	template < class ...T > inline void println_cstr(const T *...x) { print_cstr(x...), _pc(10); }
}	using FastIO::read, FastIO::read_cstr, FastIO::write, FastIO::write_cstr, FastIO::println, FastIO::println_cstr;
using B = bitset < 231 >;
int n, m, al, a[5000009], id[21][21], pre[5000009][21], dp[1 << 21], ko, o, pos, ans;
B msk[1 << 21];
namespace ST
{
	B t[5000009 << 2];
	inline int lc(int p) { return p << 1; }
	inline int rc(int p) { return p << 1 | 1; }
	inline int md(int l, int r) { return ( l + r ) >> 1; }
	inline void build(int p, int l, int r)
	{
		if ( l == r ) { t[p].set(id[a[l + 1]][a[l + 2]]); return; }
		build(lc(p), l, md(l, r)), build(rc(p), md(l, r) + 1, r), t[p] = t[lc(p)] | t[rc(p)];
	}
	inline int fnd(int p, int l, int r, int o, const B &msk)
	{
		if ( r < o || ( t[p] & msk ).none() ) return -1;
		if ( l == r ) return l;
		int z = fnd(lc(p), l, md(l, r), o, msk);
		return ~z ? z : fnd(rc(p), md(l, r) + 1, r, o, msk);
	}
}
int main()
{
	read(n);
	For(i, 1, n) read(a[i]), m = max(m, a[i]--);
	For(i, 1, n) copy(pre[i - 1], pre[i - 1] + 21, pre[i]), pre[i][a[i]] = i;
	For(i, 0, m - 1) For(j, i, m - 1) id[i][j] = id[j][i] = o++;
	ans = m, al = ( 1 << m ) - 1, ST::build(1, 0, n - 2);
	For(i, 1, al)
	{
		o = __builtin_ctz(i), msk[i] = msk[i ^ ( 1 << o )];
		For(j, 0, m - 1) if ( i & ( 1 << j ) ) msk[i].set(id[o][j]);
	}
	For(i, 0, al)
	{
		o = ST::fnd(1, 0, n - 2, dp[i], msk[i ^ al]), dp[i] = ~o ? o : n, ko = 0;
		For(j, 0, m - 1)
		{
			if ( i & ( 1 << j ) ) { ko = 1 << j; continue; }
			pos = pre[min(n, dp[i] + 2)][j];
			if ( pos ) o = ( i ^ ko ) | ( 1 << j ), dp[o] = max(dp[o], pos);
		}
	}
	For(i, 0, al) if ( dp[i] >= n - 1 ) ans = min(ans, __builtin_popcount(i));
	return println(ans), 0;
}
// 想上GM捏 想上GM捏 想上GM捏 想上GM捏 想上GM捏
// 伊娜可爱捏 伊娜贴贴捏