P22691 [USACO] Exercise
提高
题目描述

Farmer John has come up with a new morning exercise routine for the cows
(again)!

As before, Farmer John's $N$ cows ($1\le N\le 10^4$) are standing in a line.
The $i$-th cow from the left has label $i$ for each $1\le i\le N$. He tells them
to repeat the following step until the cows are in the same order as when they
started.

Given a permutation $A$ of length $N$, the cows change their order such that
the $i$-th cow from the left before the change is $A_i$-th from the left after
the change.
For example, if $A=(1,2,3,4,5)$ then the cows perform one step. If
$A=(2,3,1,5,4)$, then the cows perform six steps. The order of the cows from
left to right after each step is as follows:

0 steps: $(1,2,3,4,5)$1 step: $(3,1,2,5,4)$2 steps: $(2,3,1,4,5)$3 steps: $(1,2,3,5,4)$4 steps: $(3,1,2,4,5)$5 steps: $(2,3,1,5,4)$6 steps: $(1,2,3,4,5)$
Find the sum of all positive integers $K$ such that there exists a permutation
of length $N$ that requires the cows to take exactly $K$ steps.

As this number may be very large, output the answer modulo $M$
($10^8\le M\le 10^9+7$, $M$ is prime).

INPUT FORMAT:
The first line contains $N$ and $M$.

OUTPUT FORMAT:
A single integer.

SAMPLE INPUT:
5 1000000007
SAMPLE OUTPUT:
21

There exist permutations that cause the cows to take $1$, $2$, $3$, $4$, $5$,
and $6$ steps. Thus, the answer is $1+2+3+4+5+6=21$.

SCORING:
Test cases 2-5 satisfy $N\le 10^2$.Test cases 6-10 satisfy no additional constraints.

Problem credits: Benjamin Qi

输入格式

见题面

输出格式

见题面

样例输入
5 1000000007
样例输出
21