I'm currently reading the tutorial on linear/differential crypta here: https://www.engr.mun.ca/~howard/PAPERS/ldc_tutorial.pdf
End of page 14/Beginning of page 15: all the bits from the subkeys are summed up to a sum(k) variable. It is then written as 0 in the equation (5) and the probability stays the same.
I don't understand this step, why is the probability not changing if sum(k) is zero, and why is it 1-p if sum(k) is 1?