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I chose 3.
As 3^2 - 3 = 6 which is an even number.
Let me do it in a more general way.
Let x be the number chosen.
The calculate solution = x^2 - x
= x(x-1)

Every odd numbers can be represented 2n+1 where n is integer,
every even numbers can be represented 2k where k is integer.
For every integer, odd and even number is differed by 2n+1-2k = 2(n-k)+1
which is an odd number.

Therefore it suggested that the product of x and (x-1) must be a product of an odd and an even.

Hence the conclusion, the product of x and (x-1), x(x-1) must be an even number if x is integer.