#include<stdio.h>
#include<vector>
#include<string.h>
using namespace std;
vector<int> a;
int book[210],match[210];
int dfs(int);
int isp[100000]={0};
void su();
int main()
{
int N,i,t;
su();
for(;scanf("%d",&N)!=EOF;)
{
a.clear();
int sum=0;
for(i=0;i<N;scanf("%d",&t),a.push_back(t),i++);
memset(match,-1,sizeof(match));
for(i=0;i<a.size();i++)
{
if(a[i]%2==0){
memset(book,0,sizeof(book)); book[i]=1;
if(dfs(i)) sum++;
}
}
printf("%d\n",sum);
}
}
int dfs(int x)
{
int i;
for(i=0;i<a.size();i++)
if(a[i]%2==1&&isp[a[i]+a[x]]==0&&book[i]==0)
{
book[i]=1;
if(match[i]==-1||dfs(match[i]))
{
//printf("%d->%d\n",a[x],a[i]);
match[i]=x;
match[x]=i;
return 1;
}
}
return 0;
}
void su()
{
isp[0]=isp[1]=1;
int i,j;
for(i=2;i<100000;i++)
if(isp[i]==0){
for(j=2*i;j<100000;j+=i) isp[j]=1;
}
}