Choccy Break

Rui has a chocolate bar that is arranged into the shape of a $m \times n$ grid of $1 \times 1$ squares. Rui can break up the chocolate bar in any way he wants horizontally or vertically at each step. The goal is to obtain the constituent $mn$ $1 \times 1$ squares. What is the minimum number of breaks Rui must perform? Report the answer for $m = 10$ and $n = 8$.