Count Good Numbers - Exponent Calculation

How many even positions? Index 0, 2, 4, ... up to $n-1$. If $n$ is even, you have $n/2$ of each. If $n$ is odd, even positions get one extra.

Formula: even positions = $\lceil n/2 \rceil$, odd positions = $\lfloor n/2 \rfloor$. In code, ceiling is $(n + 1) / 2$ (integer division), floor is $n / 2$.

Check: $n = 4$ gives $5^2 \times 4^2$. $n = 5$ gives $5^3 \times 4^2$. Matches expectations.