public static boolean prime(long a){
   if(a == 2){
      return true;
   }else if(a <= 1 || a % 2 == 0){
      return false;
   }
   long max = (long)Math.sqrt(a);
   for(long n= 3; n <= max; n+= 2){
      if(a % n == 0){ return false; }
   }
   return true;
}