1 #include <iostream>
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <cstdlib>
  5 #include <string>
  6 #include <vector>
  7 #include <map>
  8 #include <set>
  9 #include <queue>
 10 #include <sstream>
 11 #include <algorithm>
 12 using namespace std;
 13 #define pb push_back
 14 #define mp make_pair
 15 #define ms(a, b)  memset((a), (b), sizeof(a))
 16 //#define LOCAL
 17 #define eps 0.0000001
 18 #define LNF (1<<60)
 19 typedef long long LL;
 20 const int inf = 0x3f3f3f3f;
 21 const int maxn = 100+10;
 22 const int mod = 1e9+7;
 23 LL a[maxn];
 24 bool Map[maxn][maxn], vis[maxn];
 25 int lin[maxn];
 26 LL big_rand(LL m)
 27 {
 28     LL x = rand();
 29     x*=rand();
 30     if(x<0) x-=x;
 31     return x%=m;
 32 }
 33 LL mod_mul(LL x, LL y, LL n)
 34 {
 35     if(x == 0 || y == 0)    return 0;
 36     return (((x&1)*y)%n+(mod_mul(x>>1, y, n)<<1)%n)%n;
 37 }
 38 LL mod_exp(LL x, LL y, LL n)
 39 {
 40     LL ret = 1;
 41     while(y){
 42         if(y&1) ret = mod_mul(ret, x, n);
 43         x = mod_mul(x, x, n);
 44         y >>= 1;
 45     }
 46     return ret;
 47 }
 48 bool Miller_Rabbin(LL n)
 49 {
 50     LL i, j, x, m, k;
 51     if(n==2)    return true;
 52     if(n<2|| !(n&1))    return false;
 53     m = n - 1;k = 0;
 54     while(!(m&1))   m >>= 1, k++;
 55     for(i=0;i<4;i++){
 56         x = big_rand(n-2) + 2;
 57         x = mod_exp(x, m, n);
 58         if(x == 1)  continue;
 59         for(j = 0;j<k;j++){
 60             if(x==n-1)  break;
 61             x = mod_mul(x, x, n);
 62         }
 63         if(j>=k)    return false;
 64     }
 65     return true;
 66 }
 67 bool dfs(int x, int n){
 68     for(int j = 1;j<=n;j++){
 69         if(Map[x][j]&&!vis[j]){
 70             vis[j] = 1;
 71             if(lin[j]==0 || dfs(lin[j], n)){
 72                 lin[j] = x;
 73                 return 1;
 74             }
 75         }
 76     }
 77     return 0;
 78 }
 79 int main()
 80 {
 81 #ifdef LOCAL
 82     freopen("input.txt", "r", stdin);
 83 //      freopen("output.txt", "w", stdout);
 84 #endif // LOCAL
 85 
 86     int n;
 87     while(~scanf("%d", &n)){
 88         ms(Map, 0);
 89         for(int i=1;i<=n;i++)    scanf("%lld", &a[i]);
 90         for(int i=1;i+1<=n;i++){
 91             for(int j=i+1;j<=n;j++){
 92                 if(Miller_Rabbin(a[i]+a[j])){
 93                     Map[i][j] = Map[j][i] = 1;
 94                 }
 95             }
 96         }
 97         int ans = 0;
 98         ms(lin, 0);
 99         for(int i=1;i<=n;i++){
100             ms(vis, 0);
101             if(dfs(i, n))  ans++;
102         }
103         printf("%d\n", ans/2);
104     }
105     return 0;
106 }