A rectangle of chocolate may seem like a calculation problem, but the right question is not how long the bar is: it is whether the first player can guarantee victory, and why.
The cursed chocolate
Riddle statement
A rectangular bar of chocolate is divided into squares, and the square in the upper left corner is poisoned. Two players take turns: on each turn, the player chooses a square and eats that square along with all the squares below and to the right of it.
The player who is forced to eat the poisoned square loses. If both play perfectly, who has a winning strategy?