18. In the Seven-Star Country, each citizen has an ID code, which is a seven-digit number composed of the digits $1 \sim 7$ without repetition. The ID code of Citizen 1 is 1234567, the ID code of Citizen 2 is 1234576, the ID code of Citizen 3 is 1234657, and the citizen numbers and ID codes increase sequentially. What is the ID code of Citizen 2520? $\qquad$ .
$4376521$