9. How many six-digit numbers exist where each digit in an even position is one greater than the digit to its left (digits are numbered from left to right)?
9. \(8 \cdot 9 \cdot 9=648\). Hint. The six-digit numbers in question are determined by the choice of the first, third, and fifth digits. (Kvant, 1972, No. 5, p. 81; MSU, 1971)