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                                                                                                                                
                                                                                                                                       
                                                                                                                                       
(1...n).each do |i|                                                                                                                    
  # i == 1 の時はn-1                                                                                                                   
  # i == 2 の時は(n-1)/2                                                                                                               
  ans += (n-1)/i                                                                                                                       
end                                                                                                                                    
print ans