102997: [Atcoder]ABC299 Ex - Dice Sum Infinity
Description
Score : $600$ points
Problem Statement
Takahashi has an unbiased six-sided die and a positive integer $R$ less than $10^9$. Each time the die is cast, it shows one of the numbers $1,2,3,4,5,6$ with equal probability, independently of the outcomes of the other trials.
Takahashi will perform the following procedure. Initially, $C=0$.
- Cast the die and increment $C$ by $1$.
- Let $X$ be the sum of the numbers shown so far. If $X-R$ is a multiple of $10^9$, quit the procedure.
- Go back to step 1.
Find the expected value of $C$ at the end of the procedure, modulo $998244353$.
Notes
Under the constraints of this problem, it can be shown that the expected value of $C$ is represented as an irreducible fraction $p/q$, and there is a unique integer $x\ (0\leq x\lt998244353)$ such that $xq \equiv p\pmod{998244353}$. Print this $x$.
Constraints
- $0\lt X\lt10^9$
- $X$ is an integer.
Input
The input is given from Standard Input in the following format:
$X$
Output
Print a single line containing the answer.
Sample Input 1
1
Sample Output 1
291034221
The expected value of $C$ at the end of the procedure is approximately $833333333.619047619$, and $291034221$ when represented modulo $998244353$.
Sample Input 2
720357616
Sample Output 2
153778832