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 Python (3.8.2) (judge verdict: Accepted)

n = int(input())

ans = 0
for a in range(1, n + 1):
    q = n // a
    r = n % a

    # a <= b
    # b: a, a+1, ..., q
    # c: ...  ,r + a, r
    if q < a:
        continue
    
    else:
        # a < b, a == b
        ans += ((q - a) * 2 + 1)
        if r == 0:
            if a == q:
                ans -= 1
            else:
                ans -= 2
        # print(a, ans)

print(ans)