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USACO 2020 US Open Contest, Gold

Problem 3. Exercise


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

As before, Farmer John's N cows (1N104) are standing in a line. The i-th cow from the left has label i for each 1iN. 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 Ai-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 (108M109+7, M is prime).

INPUT FORMAT (file exercise.in):

The first line contains N and M.

OUTPUT FORMAT (file exercise.out):

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 N102.
  • Test cases 6-10 satisfy no additional constraints.

Problem credits: Benjamin Qi

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