def prime(a)
  b = Array.new
  k = 0
  for i in 1..a.length-1
    if a[i] == 2 or a[i] == 3
      b[k] = a[i]
      k += 1
      next
    elsif a[i] % 2 != 0
      j = 3
      while a[i] % j != 0
        j += 2
        if j >= a[i]/2
          b[k] = a[i]
          k += 1
          break
        end
      end
    end
  end
  print(b)
  return k
end

