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