Problem:
Score : 300 points

Problem Statement

Given is a positive integer N.
How many tuples (A,B,C) of positive integers satisfy A \times B + C = N?

Constraints

2 \leq N \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

3

Sample Output 1

3

There are 3 tuples of integers that satisfy A \times B + C = 3: (A, B, C) = (1, 1, 2), (1, 2, 1), (2, 1, 1).

Sample Input 2

100

Sample Output 2

473

Sample Input 3

1000000

Sample Output 3

13969985

Solution in Ruby (2.7.1) (judge verdict: Runtime Error)

# https://atcoder.jp/contests/abc179/tasks/abc179_e
num, a, dev = gets.chomp.split.map(&:to_i)
ans[0] = a
a %= dev
for i in 1..num do
  a = a ** 2
  a %= dev
  if ans.include?(a)
    unit = ans[ans.index(a) .. -1]
    unit_times = (num - i) / unit.size
    units_sum = unit.sum * unit_times
    rest = unit[0 .. ((num - i) / unit.size)].sum
    ans[i] = units_sum + rest
    break
  end
  ans[i] = a
end
puts ans.sum

# a を配列に入れて、繰り返しを見つけなきゃいけない