Among the four-digit numbers that can be formed by the numbers 1, 2, 3, and 4, how many four-digit numbers have the thousand digit is 1 and there are no repeated numbers.
数字1、2、3、4可以组成的四位数中，千位数字是1的并且没有重复数字的四位数共有多少个．
<result>6</result>