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)

n = gets.chomp.to_i
ans = 0
sqrt = Math.sqrt(n).to_i
1.upto(sqrt) do |a|
    maxb = n / a
    a.upto(maxb) do |b|
        if a * b < n then
            # puts "a, b, c = #{a}, #{b}, #{n - (a * b)}"
            if a == b then
                ans += 1
            else
                ans += 2
            end
        end
    end
end
puts ans