#abc280e. [abc280_e]Critical Hit
[abc280_e]Critical Hit
Problem Statement
There is a monster with initial stamina .
Takahashi repeatedly attacks the monster while the monster's stamina remains or greater.
An attack by Takahashi reduces the monster's stamina by with probability and by with probability .
Find the expected value, modulo (see Notes), of the number of attacks before the monster's stamina becomes or less.
Notes
We can prove that the sought expected value is always a finite rational number. Moreover, under the Constraints of this problem, when the value is represented as by two coprime integers and , we can show that there exists a unique integer such that and . Print such .
Constraints
- All values in the input are integers.
Input
The input is given from Standard Input in the following format:
Output
Find the expected value, modulo , of the number of Takahashi's attacks.
Sample Input 1
3 10
Sample Output 1
229596204
An attack by Takahashi reduces the monster's stamina by with probability and by with probability .
- The monster's initial stamina is .
- After the first attack, the monster's stamina is with probability and with probability .
- After the second attack, the monster's stamina is with probability , with probability , and with probability . With probability , the stamina becomes or less, and Takahashi stops attacking after two attacks.
- If the stamina remains after two attacks, the monster's stamina always becomes or less by the third attack, so he stops attacking after three attacks.
Therefore, the expected value is $2\\times \\frac{19}{100}+3\\times\\left(1-\\frac{19}{100}\\right)=\\frac{281}{100}$. Since , print .
Sample Input 2
5 100
Sample Output 2
3
Takahashi's attack always reduces the monster's stamina by . After the second attack, the stamina remains , so the third one is required.
Sample Input 3
280 59
Sample Output 3
567484387