The effect

How it works

This card's effect allows you to summon any "Number" Xyz monster from the Extra Deck (except Number C88, who can only be Special Summoned via Rank-Up on Number 88).

In order to do so, you have to choose exactly 4 other "Number" cards, and make sure the sum of the number written in their name (which we'll call "value") equals exactly the value of the card you wanna summon.
(Some cards do not have written number per say, but have a line of text stating they are always treated as a "Number" with said value)
However, you also have to make sure that each "Number" card have a different Rank (which can variate between cards of the same value)

Translated in math form, it reads : ∀ n ∈ ℕ, ∀(n1 , n2 , n3 , n4) ∈ (0,n), n = n1 + n2 + n3 + n4

For the app itself, this means that each time you chose a Number, you can reduce the range of values from which the user can choose from for the other cards.
As an example, if n = 39, if you chose n1 = 10, you would only be able to choose between 0 and 29 for n2.
This ensures you always chose correct numbers, and also shows you when it won't fit (due to not having a valid value and rank to complete the list).