Count Good Numbers - Counting Pattern

The key insight: positions alternate between even digits and prime digits. Even positions have 5 choices each. Odd positions have 4 choices each.

For $n$ digits, count how many even positions and how many odd positions you have. If $n = 5$, positions 0, 2, 4 are even (3 positions), positions 1, 3 are odd (2 positions).

Total good numbers = $5^{\text{even positions}} \times 4^{\text{odd positions}}$. For $n = 5$, that's $5^3 \times 4^2 = 125 \times 16 = 2000$.