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)

require 'set'

n = gets.chop.to_i

a = n - 1
b = n - 1
cnt = 0
memo = []

(1..a).each do |i|
    (1..b).each do |j|
        break if memo.include?(Set[i, j])
        if (i * j) < n
            #p "i:#{i} j:#{j}"
            cnt += 1
        else
            memo << Set[i, j]
            break
        end
    end
end

puts cnt