Proving the additive inverse of a sum is the sum of the additive inverses

Question

I have this proposition to prove: For all $m, n, \in\mathbb Z$: $-(m + n) = (-m) + (-n)$ Proof: \begin{align*} -(m + n) + (m + n) &= 0 + 0\\ -(m + n) + (m + n) &= (-m) + m + (-n) + n\\ (-m) + (-n) + -(m + n) + m + n &= (-m) + (-n) + m + (-m) + (-n) + n\\ -(m + n) &= (-m) + (-n) \end{align*}

What do you think? I am a bit unsure when I go from (m + n) to m + n (remove the parentheses). Thank you!