def calc_prime(prime,num):

     i,gab=7,2

     while num>3:

         flag=True

         for x in prime:

             if x*x>i:

                 break

             if i%x==0:

                 flag=False

                 break

         if flag:

             prime.append(i)

             if len(prime)>=num:

                 break

         gab=6-gab

         i+=gab

     return prime[num-1]

 #调用

 print calc_prime([2,3,5],1000)