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: Accepted)

class InputLine
  def self.to_s
    gets.chomp
  end

  def self.to_i
    InputLine.to_s.to_i
  end
  
  def self.to_f
    InputLine.to_s.to_f
  end

  def self.to_sa
    InputLine.to_s.split(' ')
  end

  def self.to_ia
    InputLine.to_sa.map(&:to_i)
  end
  
  def self.to_fa
    InputLine.to_sa.map(&:to_f)
  end
end

def main(argv)
  n = InputLine.to_i

  s = 0
  (1..n).each do |a|
    s += n / a
    s -= 1 if n % a == 0
  end

  puts s.to_s
end

if self.to_s == 'main' then
  main(ARGV)
end