I3.1 Given that the unit digit of $1+2+3+\ldots+1997+1998+1999+1998+1997+\ldots+3+2+1$ is $P$, find the value of $P$.
$\begin{aligned} & 1+2+3+\ldots+1997+1998+1999+1998+1997+\ldots+3+2+1 \\ = & 2(1+2+\ldots+1998)+1999 \\ = & (1+1998) \times 1998+1999 \\ P= & \text { unit digit }=1\end{aligned}$