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: Time Limit Exceeded)

N = gets.to_i

list = []

(1..10**6-1).each do |a|
  (a..10**6-1).each do |b|
    break if a * b >= N
    c = N - a * b
    list << [a, b, c]
  end
end

add_list = []
list.each do |l|
  next if l[0] == l[1]
  add_list << [l[1], l[0], l[2]]
end

puts list.count + add_list.count