【Answer】Solution: $1+2+3+4+5+6+7+8+9=45$,
Therefore, the number at the bottom right cannot be less than: $45 \div 4=11 \cdots 1$, so the number is at least 12, for example: Hence the answer is: 12.
8. (15 points) Fill in the 9 cells of the 3x3 grid with 9 different natural numbers, such that: in each row, the sum of the two left numbers equals the rightmost number; in each column, the sum of the two top numbers equals the bottom number. The smallest number in the bottom-right corner is . $\qquad$