Calculate the single-digit result obtained from the repeated sum of the digits of the number $2^{2004}$. Follow these steps:
1. Calculate the sum $S$ of all digits of $2^{2004}$.
2. Calculate the sum of the digits of $S$ to obtain $R$.
3. Continue calculating the sum of the digits until a single-digit number is reached.

For example, for $2^7 = 128$, we have:
- $S = 1 + 2 + 8 = 11$
- $R = 1 + 1 = 2$
Thus, the single-digit result for $2^7$ is $2$.

Let's think step by step and output the final answer within \boxed{}.
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