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 'prime'

def divisor_count(n)
  return 1 if n == 1
  Prime.prime_division(n).map {|e| e[1] + 1 }.inject(:*)
end

n = gets.to_i

count = 0
for i in 1..n-1 do
  count += divisor_count(i)
end

puts count