How to split a chocolate bar

Take two positive integers $a,b$ and consider a rectangle of size $a\times b$ in a grid of mesh size $1\times 1$ . It models our chocolate bar, and the cells of the mesh are the chocolate pieces.

Say that a "cut" of this bar consist in choosing a piece and a vertical or horizontal grid line, then splitting that piece into two smaller pieces along this line.

What is the minimal number of cuts needed to completely separate the chocolate bar into its $ab$ pieces?

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Solution by Ancient Tree

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Each cut increases the number of pieces by 1, no matter how you want to cut it ! Starting from a single piece, there needs to be $ab-1$ cuts to reach $ab$ pieces.

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Solution by unkempt6571FR

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On découpe d’abord en $a$ colonnes (de dimensions $b\times1$ ) ce qui nécessite $a-1$ découpes.
Puis, on découpe chaque colonne en carrés de dimensions $1\times 1$ , ce qui nécessite $b-1$ découpes pour chaque colonne.
On a donc réalisé au total : $a-1 + a(b-1) = a - 1 + ab - a = ab -1$ découpes

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