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 Common Lisp (SBCL 2.0.3) (judge verdict: Accepted)

(let ((n (read))
    (a 0)
    (ans 0))
    (setq a n)
    (loop for i from 1 to n do
        (loop for j from a downto 1 do
            (if (< (* i j) n)
                (progn
                    ;; (format t "~D ~D ~%" i j)
                    (incf ans j)
                    (setq a j)
                    (return)
                )
                ;; (format t "NO~D ~D ~%" i j)
            )
        
        )
    
    )
    (format t "~D~%" ans)
)