题目大意：

 

解题：

#include 
     
      #include 
      
       using namespace std;int n ,k ;int arr[21];int res;bool isPrime(int a){    for(int i = 2; i < sqrt(a)+1; i++)        if(a % i == 0)            return false;    return true;}void search(int start,int count,int total){    if(count == k && isPrime(total))        res++;    if(count == k && isPrime(total) == false)        return;    if(count > k )        return;    for(int i = start; i < n; i++)    {        total += arr[i];        count++;        search(i+1,count,total);        count--;        total -=arr[i];    }}int main(){    cin >> n >> k;    for(int i = 0; i < n; i++)    {        cin >> arr[i];    }    res = 0;    search(0,0,0);    cout << res << endl;    return 0;}