A10231 · Wet Shark and Flowers

题目描述

There are $n$ sharks who grow flowers for Wet Shark. They are all sitting around the table, such that sharks $i$ and $i+1$ are neighbours for all $i$ from $1$ to $n-1$ . Sharks $n$ and $1$ are neighbours too. Each shark will grow some number of flowers $s_{i}$ . For $i$ -th shark value $s_{i}$ is random integer equiprobably chosen in range from $l_{i}$ to $r_{i}$ . Wet Shark has it's favourite prime number $p$ , and he really likes it! If for any pair of neighbouring sharks $i$ and $j$ the product $s_{i}·s_{j}$ is divisible by $p$ , then Wet Shark becomes happy and gives $1000$ dollars to each of these sharks. At the end of the day sharks sum all the money Wet Shark granted to them. Find the expectation of this value.

The first line of the input contains two space-separated integers $n$ and $p$ ( $3<=n<=100000,2<=p<=10^{9}$ ) — the number of sharks and Wet Shark's favourite prime number. It is guaranteed that $p$ is prime. The $i$ -th of the following $n$ lines contains information about $i$ -th shark — two space-separated integers $l_{i}$ and $r_{i}$ ( $1<=l_{i}<=r_{i}<=10^{9}$ ), the range of flowers shark $i$ can produce. Remember that $s_{i}$ is chosen equiprobably among all integers from $l_{i}$ to $r_{i}$ , inclusive.

Print a single real number — the expected number of dollars that the sharks receive in total. You answer will be considered correct if its absolute or relative error does not exceed $10^{-6}$ . Namely: let's assume that your answer is $a$ , and the answer of the jury is $b$ . The checker program will consider your answer correct, if ![](/uploads/acgo/image/baa19e8e2cf28be4_36a94b35b08a.jpeg).

输入 #1

3 2
1 2
420 421
420420 420421

输出 #1

4500.0

输入 #2

输出 #2

0.0