How many nonnegative integers can be written in the form $a_7\cdot3^7 + a_6\cdot3^6 + a_5\cdot3^5 + a_4\cdot3^4 + a_3\cdot3^3 + a_2\cdot3^2 + a_1\cdot3^1 + a_0\cdot3^0$, where $a_i \in \{-1,0,1\}$ for $0 \le i \le 7$?

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