Олимпиадный тренинг

Задача 39085. Let's count the numbers - 07


The value of an arithmetic expression
\(2 \cdot 8^{99} + 2\cdot8 ^{30}- 2\cdot8^{85} - 2 \cdot 8^{15}+3\cdot 8^{5} -8^3- 2 \)
written in the number system with base 8. How many 7 digits does this entry contain?