#include<bits/stdc++.h>

typedef unsigned int uint;
typedef long long ll;
typedef unsigned long long ull;
typedef double lf;
typedef long double llf;
typedef std::pair<int,int> pii;

#define xx first
#define yy second

template<typename T> inline T max(T a,T b){return a>b?a:b;}
template<typename T> inline T min(T a,T b){return a<b?a:b;}
template<typename T> inline T abs(T a){return a>0?a:-a;}
template<typename T> inline bool repr(T &a,T b){return a<b?a=b,1:0;}
template<typename T> inline bool repl(T &a,T b){return a>b?a=b,1:0;}
template<typename T> inline T gcd(T a,T b){T t;if(a<b){while(a){t=a;a=b%a;b=t;}return b;}else{while(b){t=b;b=a%b;a=t;}return a;}}
template<typename T> inline T sqr(T x){return x*x;}
#define mp(a,b) std::make_pair(a,b)
#define pb push_back
#define I inline
#define mset(a,b) memset(a,b,sizeof(a))
#define mcpy(a,b) memcpy(a,b,sizeof(a))

#define fo0(i,n) for(int i=0,i##end=n;i<i##end;i++)
#define fo1(i,n) for(int i=1,i##end=n;i<=i##end;i++)
#define fo(i,a,b) for(int i=a,i##end=b;i<=i##end;i++)
#define fd0(i,n) for(int i=(n)-1;~i;i--)
#define fd1(i,n) for(int i=n;i;i--)
#define fd(i,a,b) for(int i=a,i##end=b;i>=i##end;i--)
#define foe(i,x)for(__typeof((x).end())i=(x).begin();i!=(x).end();++i)

struct Cg{I char operator()(){return getchar();}};
struct Cp{I void operator()(char x){putchar(x);}};
#define OP operator
#define RT return *this;
#define RX x=0;char t=P();while((t<'0'||t>'9')&&t!='-')t=P();bool f=0;\
if(t=='-')t=P(),f=1;x=t-'0';for(t=P();t>='0'&&t<='9';t=P())x=x*10+t-'0'
#define RL if(t=='.'){lf u=0.1;for(t=P();t>='0'&&t<='9';t=P(),u*=0.1)x+=u*(t-'0');}if(f)x=-x
#define RU x=0;char t=P();while(t<'0'||t>'9')t=P();x=t-'0';for(t=P();t>='0'&&t<='9';t=P())x=x*10+t-'0'
#define TR *this,x;return x;
I bool IS(char x){return x==10||x==13||x==' ';}template<typename T>struct Fr{T P;I Fr&OP,(int&x)
{RX;if(f)x=-x;RT}I OP int(){int x;TR}I Fr&OP,(ll &x){RX;if(f)x=-x;RT}I OP ll(){ll x;TR}I Fr&OP,(char&x)
{for(x=P();IS(x);x=P());RT}I OP char(){char x;TR}I Fr&OP,(char*x){char t=P();for(;IS(t);t=P());if(~t){for(;!IS
(t)&&~t;t=P())*x++=t;}*x++=0;RT}I Fr&OP,(lf&x){RX;RL;RT}I OP lf(){lf x;TR}I Fr&OP,(llf&x){RX;RL;RT}I OP llf()
{llf x;TR}I Fr&OP,(uint&x){RU;RT}I OP uint(){uint x;TR}I Fr&OP,(ull&x){RU;RT}I OP ull(){ull x;TR}};Fr<Cg>in;
#define WI(S) if(x){if(x<0)P('-'),x=-x;char s[S],c=0;while(x)s[c++]=x%10+'0',x/=10;while(c--)P(s[c]);}else P('0')
#define WL if(y){lf t=0.5;for(int i=y;i--;)t*=0.1;if(x>=0)x+=t;else x-=t,P('-');*this,(ll)(abs(x));P('.');if(x<0)\
x=-x;while(y--){x*=10;x-=floor(x*0.1)*10;P(((int)x)%10+'0');}}else if(x>=0)*this,(ll)(x+0.5);else *this,(ll)(x-0.5);
#define WU(S) if(x){char s[S],c=0;while(x)s[c++]=x%10+'0',x/=10;while(c--)P(s[c]);}else P('0')
template<typename T>struct Fw{T P;I Fw&OP,(int x){WI(10);RT}I Fw&OP()(int x){WI(10);RT}I Fw&OP,(uint x){WU(10);RT}
I Fw&OP()(uint x){WU(10);RT}I Fw&OP,(ll x){WI(19);RT}I Fw&OP()(ll x){WI(19);RT}I Fw&OP,(ull x){WU(20);RT}I Fw&OP()
(ull x){WU(20);RT}I Fw&OP,(char x){P(x);RT}I Fw&OP()(char x){P(x);RT}I Fw&OP,(const char*x){while(*x)P(*x++);RT}
I Fw&OP()(const char*x){while(*x)P(*x++);RT}I Fw&OP()(lf x,int y){WL;RT}I Fw&OP()(llf x,int y){WL;RT}};Fw<Cp>out;

const int N=64005,M=432100;

struct edge
{
	int to,w,ne;
}e[M*2+2];

int p[N+1],p2[N+1],em=2,dep[N+1],q[N+1],qe,fl[N+1],tm[N+1],T;

inline void add(int a,int b,int w)
{
	e[em].to=b,e[em].w=w,e[em].ne=p[a],p[a]=em++;
	e[em].to=a,e[em].w=0,e[em].ne=p[b],p[b]=em++;
}

bool dfs(int s,int t)
{
	if(s==t)return 1;
	for(int&j=p2[s];j;j=e[j].ne)
		if(tm[e[j].to]!=T&&e[j].w&&dep[e[j].to]==dep[s]+1)
		{
			tm[e[j].to]=T;
			fl[e[j].to]=min(e[j].w,fl[s]);
			if(dfs(e[j].to,t))
			{
				e[j].w-=fl[t];
				e[j^1].w+=fl[t];
				return 1;
			}
		}
	dep[s]=-1;
	return 0;
}

inline int dinic(int s,int t)
{
	int flow=0;
	while(1)
	{
		memset(dep,0,sizeof(dep));
		q[0]=s,qe=1,dep[s]=1;
		++T;
		for(int i=0;i^qe;i++)
		{
			tm[q[i]]=T;
			for(int j=p[q[i]];j;j=e[j].ne)
				if(!dep[e[j].to]&&e[j].w)
				{
					dep[e[j].to]=dep[q[i]]+1;
					q[qe++]=e[j].to;
				}
		}
		if(tm[t]!=T)break;
		for(int i=0;i^qe;i++)
			p2[q[i]]=p[q[i]];
		while(1)
		{
			fl[s]=0x7fffffff;
			++T;
			dfs(s,t);
			if(tm[t]!=T)break;
			flow+=fl[t];
		}
	}
	return flow;
}

std::map<int,int>t;

bool pp(int n)
{
	if(n<=2)return 0;
	for(int i=2;i*i<=n;i++)if(n%i==0)return 0;
	return 1;
}

int main()
{
	for(int n=in,x;n--;)
	{
		in,x;
		t[x]^=1,t[x+1]^=1;
	}
	std::vector<int>s[2];
	foe(i,t)if(i->yy)s[i->xx&1].pb(i->xx);
	int a=s[0].size(),b=s[1].size(),S=a+b,T=a+b+1;
	//fo0(i,a)out,s[0][i],' ';out,'\n';
	//fo0(i,b)out,s[1][i],' ';out,'\n';
	fo0(i,a)add(S,i,1);
	//fo0(i,a)fo0(j,b)if(pp(abs(s[0][i]-s[1][j])))out,'/',i,' ',j,'\n';
	fo0(i,a)fo0(j,b)if(pp(abs(s[0][i]-s[1][j])))add(i,j+a,1);
	fo0(i,b)add(i+a,T,1);
	int g=dinic(S,T);
	//out,a,' ',b,' ',g,'\n';
	//k × 1 + (?(Me - k)/2? + ?(Mo - k)/2?) × 2 + ((Me - k)%2) × 3
	int ans=a+b-g+(a-g)%2;
	out,ans,'\n';
}

Measured memory_bytes: 640