#include<bits/stdc++.h>
#
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> P;
#define fi first
#define se second
#define mkp make_pair
#define pb emplace_back
#define popcnt __builtin_popcountll
const ll mod = 998244353;
inline ll read(){
	ll x=0, f=1; char ch=getchar();
	while(ch<'0' || ch>'9') { if(ch=='-') f=-1; ch=getchar(); }
	while(ch>='0' && ch<='9') x=x*10+ch-'0', ch=getchar();
	return x*f;
}
inline int lg2(int x){ return 31^__builtin_clz(x); }
inline ll lg2(ll x){ return 63^__builtin_clzll(x); }
inline void addmod(int &x){ if(x >= mod) x -= mod; }
inline void addmod(ll &x){ if(x >= mod) x -= mod; }
inline ll qpow(ll a,ll b){
	ll ans=1, base=a;
	while(b){
		if(b&1) ans=ans*base%mod;
		base=base*base%mod; b>>=1;
	}
	return ans;
}
inline ll INV(ll x){ return qpow(x, mod-2); };
const int M = 21;
int n,a[5000005];
int f[1<<M],pp[1<<M];

void procedure(){
	n=read();
	for(int i=1;i<=n;i++) a[i]=read()-1;
	int ans = M;
	for(int i=1;i<(1<<M);i++) pp[i]=1+pp[i^(i&(-i))];
		for(int i=0;i<(1<<M);i++){
			while(f[i] < n-1 && (((i>>a[f[i]+1])&1) || ((i>>a[f[i]+2])&1)))
				++f[i];
			if(f[i] == n-1){
				ans = min(ans, pp[i]);
			}
			for(int j=0; j<M-1; j++)
				f[(i^(1<<j))|(1<<j+1)]=max(f[(i^(1<<j))|(1<<j+1)],f[i]);
			f[i|1]=max(f[i|1], f[i]);
		}
	printf("%d\n", ans);
}
int main(){
	#ifdef LOCAL
		assert(freopen("input.txt","r",stdin));
		assert(freopen("output.txt","w",stdout));
	#endif
	ll T=1;
	// math_init();
	// NTT::init();
	while(T--) procedure();
	return 0;
}