P6521 [CEOI 2010] pin (day2)

Given $n$ strings of length $4$, you need to find how many pairs of strings have **exactly** $D$ positions where the corresponding characters are different.

The first line contains two integers $n, D$. The next $n$ lines each contain a string of length $4$.

Output one integer on a single line, representing the number of pairs of strings that satisfy the condition.

#### Sample Explanation #### Sample 1 Explanation Any two strings differ in more than one position. #### Sample 2 Explanation There are three pairs of strings: `0000,a010` `a010,a0e2` `0000,0202`. #### Constraints - For $15\%$ of the testdata, it is guaranteed that $n \le 2000$. - For $30\%$ of the testdata, it is guaranteed that $D = 1$. - For $60\%$ of the testdata, it is guaranteed that $D \le 2$. - For $75\%$ of the testdata, it is guaranteed that the strings contain only lowercase letters $a \sim f$ and digits, so they can be treated as hexadecimal numbers. - For $100\%$ of the testdata, it is guaranteed that $2 \le n \le 5 \times 10^4$, $1 \le D \le 4$, all input strings are distinct, and each character in the string is either $a \sim z$ or a digit character. #### Notes **This problem is translated from [CEOI 2010](http://ceoi2010.ics.upjs.sk/Contest/Tasks) day 2 *[T2 pin](https://people.ksp.sk/~misof/ceoi2010/pin-eng.pdf)***. The copyright of this translation belongs to the problem provider @[ShineEternal](https://www.luogu.com.cn/user/45475). Reposting without permission is prohibited. Translated by ChatGPT 5