CF1213C.Book Reading

Polycarp is reading a book consisting of $n$ pages numbered from $1$ to $n$. Every time he finishes the page with the number divisible by $m$, he writes down the last digit of this page number. For example, if $n=15$ and $m=5$, pages divisible by $m$ are $5, 10, 15$. Their last digits are $5, 0, 5$ correspondingly, their sum is $10$.

Your task is to calculate the sum of all digits Polycarp has written down.

You have to answer $q$ independent queries.

Polycarp 正在阅读一本由 $n$ 页组成的书，页码从 $1$ 到 $n$ 编号。每当他读完页码能被 $m$ 整除的页面时，他就会写下该页码的最后一位数字。例如，若 $n=15$ 且 $m=5$，则能被 $m$ 整除的页码为 $5, 10, 15$，它们的最后一位数字分别为 $5, 0, 5$，其和为 $10$。

你的任务是计算 Polycarp 所写下的所有数字之和。

你需要回答 $q$ 个相互独立的查询。

The first line of the input contains one integer $q$ ($1 \le q \le 1000$) — the number of queries.

The following $q$ lines contain queries, one per line. Each query is given as two integers $n$ and $m$ ($1 \le n, m \le 10^{16}$) — the number of pages in the book and required divisor, respectively.

输入的第一行包含一个整数 $q$（$1 \le q \le 1000$），表示查询次数。

接下来的 $q$ 行每行包含一个查询，每个查询由两个整数 $n$ 和 $m$（$1 \le n, m \le 10^{16}$）组成，分别表示书的页数和要求的因数。

For each query print the answer for it — the sum of digits written down by Polycarp.

对于每个查询，请输出对应的答案——即 Polycarp 写下的所有数字的各位数字之和。