#include <bits/stdc++.h>

using namespace std;

namespace FastRead{
	char buf[1 << 20 | 5], *s = buf, *t = buf;
	#ifdef LOCAL
	#define gc() getchar()
	#else
	#define gc() s == t && (t = (s = buf) + fread(buf, 1, 1 << 20, stdin), s == t) ? EOF : *s ++ 
	#endif
	template <typename T>
	inline void Read(T &x)
	{
		x = 0;
		char ch = gc();
		while(ch < '0' || ch > '9') ch = gc();
		while('0' <= ch && ch <= '9') x = (x * 10) + (ch ^ 48), ch = gc();
	}
}
using FastRead::Read;

const int N = 5e6 + 5, M = 1 << 21 | 5;

int n, m;
int a[N];
int f[M];

int main()
{
	Read(n);
	for(int i = 1; i <= n; i ++ )
	{
		Read(a[i]);
		m = max(m, a[i]);
		a[i] -- ;
	}
	
	int ans = m;
	for(int i = 0; i < 1 << m; i ++ )
	{
		int k = __builtin_popcount(i);
		if(k >= ans) continue;
		while(f[i] < n - 1 && (((i >> a[f[i] + 1]) | (i >> a[f[i] + 2])) & 1)) f[i] ++ ;
		if(f[i] >= n - 1) ans = k;
		for(int j = 1; j < m; j ++ )
			f[(i ^ (1 << (j - 1))) | (1 << j)] = max(f[(i ^ (1 << (j - 1))) | (1 << j)], f[i]);
		f[i | 1] = max(f[i | 1], f[i]);
	}
	
	printf("%d\n", ans);
	return 0;
}