Write a function called max$-$ops$-$diff in R$5$RS Scheme that takes a list of magnitudes $($positive integers$),$ zero excluded. Then perform the operation of either placing a $+$ or $-$ in front of each number, thus making it positive or making it negative, and getting the sum of these values. Determine the maximum difference of the number of $+$ and $-$ required for a sum of $0.$ In other words, how many positive and negative values are required to produce a sum of $0$ with the given magnitudes. If a sum of $0$ is not possible then $-1$ should be returned. All values must be used.