12. Write down the sequence of consecutive odd natural numbers starting from 1: $1357911131517 \ldots . .$. , until the number string "2014" appears for the first time. The total number of digits written is $\qquad$.
【Answer】7850
【Analysis】Key point: Page number problem
Since it is an odd number sequence, the even number "2014" cannot appear, so the first appearance of "2014" should be in a two-digit number, totaling $45 \times 2=90$ digits; (3) From 101 to 999, there are $(999-101) \div 2+1=450$ three-digit numbers, totaling $450 \times 3=1350$ digits; (4) From 1001 to 4201, there are $(4201-1001) \div 2+1=1601$ four-digit numbers, totaling $1601 \times 4=6404$ digits. $5+90+1350+6404=7849$, plus 1 more 4, totaling $7849+1=7850$ digits.