Solve in Python:
You went shopping to buy cakes and donuts with X yen (the currency of Japan).
First, you bought one cake for A yen at a cake shop.
Then, you bought as many donuts as possible for B yen each, at a donut shop.
How much do you have left after shopping?

-----Constraints-----
 - 1 \leq A, B \leq 1 000
 - A + B \leq X \leq 10 000
 - X, A and B are integers.

-----Input-----
Input is given from Standard Input in the following format:
X
A
B

-----Output-----
Print the amount you have left after shopping.

-----Sample Input-----
1234
150
100

-----Sample Output-----
84

You have 1234 - 150 = 1084 yen left after buying a cake.
With this amount, you can buy 10 donuts, after which you have 84 yen left.
X = int(input())
A = int(input())
B = int(input())

print((X - A) % B)