sum of non-prime index

import java.util.*;
public class VK{
public static void main (String []ar){
int a[]={-1,-2,-3,3,4,-7};

  int k=0;
  String s="";
 for(int i=0;i<a.length;i++)
 {  
     int c=0;
     for(int j=1;j<=i;j++)
     {
         if(i%j==0)
         
         c++;
     }
 
     if(c!=2)
     {
     s+=a[i];
     k+=a[i];
     }
 }
 System.out.println(s);
  System.out.println(k);
}

}