#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<vector>
#include<queue>
#include<map>
#include<bitset>
#include<set>
#define N (200010)
#define P ()
#define M (11000010)
#define inf (0x7f7f7f7f)
#define rg register int
#define Label puts("NAIVE")
#define spa print(' ')
#define ent print('\n')
#define rand() (((rand())<<(15))^(rand()))
#define file(s) freopen(s".in","r",stdin),freopen(s".out","w",stdout)
typedef long double ld;
typedef long long LL;
typedef unsigned long long ull;
using namespace std;
namespace fastIO1{
	inline char read(){
		static const int IN_LEN=1000000;
		static char buf[IN_LEN],*s,*t;
		return (s==t?t=(s=buf)+fread(buf,1,IN_LEN,stdin),(s==t?-1:*s++):*s++);
	}
	template<class T>
	inline void read(T &x){
		static bool iosig;
		static char c;
		for(iosig=false,c=read();!isdigit(c);c=read()){
			if(c=='-')iosig=true;
			if(c==-1)return;
		}
		for(x=0;isdigit(c);c=read())x=((x+(x<<2))<<1)+(c^'0');
		if(iosig)x=-x;
	}
	inline char readc(char &c){
		for(c=read();!isalpha(c)&&!isdigit(c);c=read())
		if(c==-1)return 0;
	}
	const int OUT_LEN = 10000000;
	char obuf[OUT_LEN],*ooh=obuf;
	inline void print(char c) {
		if(ooh==obuf+OUT_LEN)fwrite(obuf,1,OUT_LEN,stdout),ooh=obuf;
		*ooh++=c;
	}
	template<class T>
	inline void print(T x){
		static int buf[30],cnt;
		if(x==0)print('0');
		else{
			if(x<0)print('-'),x=-x;
			for(cnt=0;x;x/=10)buf[++cnt]=x%10+48;
			while(cnt)print((char)buf[cnt--]);
		}
	}
	inline void flush(){fwrite(obuf,1,ooh-obuf,stdout);}
}
namespace fastIO2{
	template<class T>
	inline void read(T &x){
		static bool iosig; 
		static char c;
		for(iosig=false,c=getchar();!isdigit(c);c=getchar()){
			if(c=='-')iosig=true;
			if(c==-1)return;
		}
		for(x=0;isdigit(c);c=getchar())x=((x+(x<<2))<<1)+(c^'0');
		if(iosig)x=-x;
	}
}
using namespace fastIO1;
int n,a[M],prime[M],sum,cnt1,cnt2,od[N],ev[N];
bool pr[M]; queue<int> Q; int dis[N],cur[N];
int fi[N],ne[N<<4],to[N<<4],c[N<<4],S,T,V,E;
bool bfs(int S,int T){
	while(!Q.empty())Q.pop(); Q.push(S);
	for(int i=1;i<=V;i++)dis[i]=inf,cur[i]=fi[i]; dis[S]=0;
	while(!Q.empty()){
		int u=Q.front();Q.pop();
		for(int i=fi[u];i;i=ne[i])
		if(c[i]&&dis[to[i]]==inf){
			int v=to[i];
			dis[v]=dis[u]+1;
			if(v==T)return 1; Q.push(v);
		}
	}
	return dis[T]<=V;
}
int dfs(int u,int T,int flow){
	if(u==T)return flow;
	int usd=0;
	for(int i=cur[u];i;i=cur[u]=ne[i])
	if(c[i]&&dis[to[i]]==dis[u]+1){
		int fl=dfs(to[i],T,min(flow-usd,c[i]));
		if(!fl)continue;c[i]-=fl,c[i^1]+=fl,usd+=fl;
		if(usd>=flow)return usd;
	}
	return usd;
}
int dinic(int S,int T){
	int ans=0;
	while(bfs(S,T))ans+=dfs(S,T,inf);
	return ans;	
}
void add(int x,int y,int z){
	ne[++E]=fi[x],fi[x]=E,to[E]=y,c[E]=z;
	ne[++E]=fi[y],fi[y]=E,to[E]=x,c[E]=0;
}
void Eular(int n){
	pr[1]=1;
	for(int i=2;i<=n;i++){
		if(!pr[i]){
			prime[++sum]=i;
			if(i==2)pr[i]=1;
		}
		for(int j=1;j<=sum;j++){
			if(prime[j]*i>n)break;
			pr[prime[j]*i]=1;if(i%prime[j]==0)break;
		}
	}
}
int main(){
	read(n),E=1;
	Eular(10000010);
	for(int i=1;i<=n;i++){
		int x; read(x);
		a[x]=1;
	}
	for(int i=1;i<=10000001;i++)
	if(a[i]!=a[i-1]){if(i&1)od[++cnt1]=i;else ev[++cnt2]=i;}
	S=cnt1+cnt2+1,T=S+1,V=T;
	for(int i=1;i<=cnt1;i++){
		add(S,i,1);
		for(int j=1;j<=cnt2;j++){
			int d=(int)abs(od[i]-ev[j]);
			if(!pr[d])add(i,j+cnt1,1);
			if(i==1)add(j+cnt1,T,1);
		}
	} 
	int ans=dinic(S,T); cnt1-=ans,cnt2-=ans;
	cout<<ans+(cnt1/2)*2+(cnt2/2)*2+((cnt1&1)*3);
}

Measured memory_bytes: 65792