How to calculate minimum number of operations to make a number divisible by another

Question

I have a coding prompt I am trying to solve:

You are given two positive integers a and b. In one move you can increase a by 1 (replace a with a+1). Your task is to find the minimum number of moves you need to do in order to make a divisible by b. It is possible that you have to make 0 moves, as a is already divisible by b. You have to answer t independent test cases.

Input: The first line of the input contains t(1<t<10000) - the number of test cases then t test case follows the only line of the test case contains two integers a,b(1<=a,b<=10^9)

Output: For each test case print the answer the minimum number of moves you need to do in order to a divisible by b

Here is my code:

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
  long long int t, a, b;
  int value, count = 0, i, j;
  scanf("%lld", &t);
  if (t >= 1 && t <= pow(10, 4))
  {
    for (j = 1; j <= t; j++)
    {
      scanf("%lld %lld", &a, &b);
      if ((a >= 1 && a <= pow(10, 9)) && (b >= 1 && b <= pow(10, 9)))
      {
        value = a;
        for (i = 1; a % b != 0; i++)
        {
          count++;
          a = (value + i);
        }

        printf("%d\n", count);
        a = 0;
        b = 0;
        count = 0;
      }
    }
  }
  return 0;
}

Answer 1

( With How do I ask and answer homework questions? in mind I turn my comment into the first step of an answer according to the compromise desribed there. )

Hint 1: Output the input values and a % b (in case of hidden test cases just invent a few). What does that value tell you about the number of moves you will need?