Project Euler 第35题：

The number, 197, is called a circular prime because all rotations of the digits: 197, 971, and 719, are themselves prime.

There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.

How many circular primes are there below one million?

circular primes ,循环的素数。指的是一个素数每次把第一位移到最后一位后面,形成新的数仍然为素数。比如197，971，719。

求1000，000以内有多少循环的素数circular primes？