sum of prime index

import java.util.*;
public class VK{
public static void main (String []ar){
int a[]={10,20,30,40,50,60,70,80,90,100 };
int b=0;
int k=0;
String s="";
for(int i=2;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);
}

}