Using the digits $1,2,3,4,5,6,7,8,9$ to form all possible four-digit numbers without repeating any digit, the sum of these numbers is $\qquad$.
12. 16798320 .

Yaran has $9 \times 8 \times 7 \times 6$ four-digit numbers that meet the requirements. For any such four-digit number, there is a unique corresponding different four-digit number such that the sum of the digits in the corresponding positions of these two four-digit numbers is 10. Therefore, there are $\frac{1}{2} \times 9 \times 8 \times 7 \times 6$ pairs of numbers. The sum of each pair is $10^{3} \times 10+10^{2} \times 10+10 \times 10+10=11110$. Therefore, the required sum is $\frac{1}{2} \times 9 \times 8 \times$ $7 \times 6 \times 11110=16798320$.