Wrong Addition
Contest: Codeforces Round 762 (Div. 3)
Difficulty rating: 1200
Tags: implementation

Tanya is learning how to add numbers, but so far she is not doing it correctly. She is adding two numbers $$$a$$$ and $$$b$$$ using the following algorithm:

1. If one of the numbers is shorter than the other, Tanya adds leading zeros so that the numbers are the same length.
2. The numbers are processed from right to left (that is, from the least significant digits to the most significant).
3. In the first step, she adds the last digit of $$$a$$$ to the last digit of $$$b$$$ and writes their sum in the answer.
4. At each next step, she performs the same operation on each pair of digits in the same place and writes the result to the left side of the answer.

For example, the numbers $$$a = 17236$$$ and $$$b = 3465$$$ Tanya adds up as follows:

$$$$$$ \large{ \begin{array}{r} + \begin{array}{r} 17236\\ 03465\\ \end{array} \\ \hline \begin{array}{r} 1106911 \end{array} \end{array}} $$$$$$

- calculates the sum of $$$6 + 5 = 11$$$ and writes $$$11$$$ in the answer.
- calculates the sum of $$$3 + 6 = 9$$$ and writes the result to the left side of the answer to get $$$911$$$.
- calculates the sum of $$$2 + 4 = 6$$$ and writes the result to the left side of the answer to get $$$6911$$$.
- calculates the sum of $$$7 + 3 = 10$$$, and writes the result to the left side of the answer to get $$$106911$$$.
- calculates the sum of $$$1 + 0 = 1$$$ and writes the result to the left side of the answer and get $$$1106911$$$.

As a result, she gets $$$1106911$$$.

You are given two positive integers $$$a$$$ and $$$s$$$. Find the number $$$b$$$ such that by adding $$$a$$$ and $$$b$$$ as described above, Tanya will get $$$s$$$. Or determine that no suitable $$$b$$$ exists.

Input format:
The first line of input data contains an integer $$$t$$$ ($$$1 \le t \le 10^4$$$) — the number of test cases.

Each test case consists of a single line containing two positive integers $$$a$$$ and $$$s$$$ ($$$1 \le a \lt s \le 10^{18}$$$) separated by a space.

Output format:
For each test case print the answer on a separate line.

If the solution exists, print a single positive integer $$$b$$$. The answer must be written without leading zeros. If multiple answers exist, print any of them.

If no suitable number $$$b$$$ exists, output -1.

Constraints:
Time limit: 1.0 s
Memory limit: 256.0 MB

Note:
The first test case is explained in the main part of the statement.

In the third test case, we cannot choose $$$b$$$ that satisfies the problem statement.

Example input:
6
17236 1106911
1 5
108 112
12345 1023412
1 11
1 20

Example output:
3465
4
-1
90007
10
-1

Editorial:
Let's compute the answer to the array $$$b$$$, where $$$b_ k$$$ is the digit at the $$$k$$$ position in the number we are looking for. Let $$$i$$$ be the position of the last digit in number $$$a$$$, $$$j$$$ be the position of the last digit in number $$$s$$$. Then denote $$$x = a_i$$$, $$$y = s_j$$$, and consider the cases:
• if $$$x \le y$$$, then the sum of $$$a_i + b_i$$$ was exactly $$$s_i$$$, then $$$b_i = y - x$$$.
• if $$$x \gt y$$$, then the sum $$$a_i + b_i$$$ was greater than $$$9$$$ and we need to look at the next digit of the number $$$s$$$. If there isn't one, we can't get the answer — we'll output -1. Otherwise we recalculate $$$y = 10 \cdot s_{j - 1} + s_j$$$ and reduce $$$j$$$ by one.
• if now $$$y \ge 10$$$ and $$$y \le 18$$$, then $$$b_i = y - x$$$. Otherwise, we deduce -1, since we cannot get more than $$$9 + 9 = 18$$$ when adding two digits, and the cases where $$$a_i + b_i \lt 10$$$ have already been considered before.