Randomly write a 30-digit number consisting of numbers 1, 2, and 3, and randomly intercept three adjacent digits from this 30-digit number to obtain a three-digit number. Please prove that: Of all three-digit numbers, two must be equal.
任意写一个由数字l、2、3组成的三十位数，从这个三十位数中任意截取相邻三位，可得一个三位数，请证明：在从各个不同位置上截得的所有三位数中，一定有两个相等．
<result>28</result>