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 $1234657, \cdots \cdots$, and the citizen numbers and ID codes increase sequentially. What is the ID code of Citizen 2520? $\qquad$
\begin{tabular}{|c|}
\hline 18 \\
\hline 4376521 \\
\hline
\end{tabular}