The Frobenius number for two coprime positive integers $a$ and $b$ is $g(a,b) = ab - a - b$, the largest integer that CANNOT be represented as $ax + by$ for non-negative integers $x, y$. For $a = 6$ and $b = 11$, what is the largest number of items that CANNOT be purchased if items come in packs of 6 or 11?